Dogs need to chew.
The foster mutts that come through WAGS, being mainly puppies, adolescents, and young adult dogs, especially need to chew. Kongs are and will always be my #1 favorite dog toy, but they don't satisfy the need to gnaw. So you'll need to provide some chew toys, because that arsenal of rawhides and fake bones is all that keeps your furniture (sorta) safe.
But chew toys are expensive, and not all of them are safe, and some aren't really "chew toys" at all, depending on the vigor and determination with which your foster pup destroys things.
Not Chew Toys: Ropes, tennis balls, soft rubber squeaky toys. All these toys can be great fun for dogs, but you should always supervise a mutt who has them, because they're easily torn apart (yes, even the tennis balls -- I've met dogs at the park who could shuck the green coating off a tennis ball faster than I could peel a banana, and then split the ball inside with one chomp, creating not one but three spiffy little choking hazards). Rope toys in particular seem to get shredded into dental floss really quickly... and then whoever did the shredding tries to eat them, potentially resulting in some very expensive vet bills.
Only Kinda Chew Toys: Bully sticks, tendons, pig ears, and dried jerky treats. They're fine treats (within reason -- pig ears have a lot of calories!), but in my opinion they just don't last long enough to be worth the name. I've never had one last longer than a minute and a half.
Chew Toys: Everything that follows.
As a preface, given the concerns about the safety of imported pet foods and treats (which, as of this writing, have recently flared up again with respect to chicken jerky treats from China), I would personally look for toys made in the U.S. or Canada (and, where relevant, from animals raised in the U.S. or Canada) and avoid anything made in China.
Antlers: A relatively recent arrival on the chew toy scene, these are deer and elk antlers sawn into standard lengths. Most of the dogs I've given them to have been intrigued by the antlers because of their lingering animal smell, but not hugely possessive of them, so I'd consider them medium-value toys. I stopped providing antlers after hearing one too many horror stories about dogs cracking teeth on them; they are very hard, and probably ill-advised for a vigorous chewer who likes to really crack down on her teeth. But for a less intense chewer they should be fine. They last a long, long time.
Cheese Chews: Rock-hard plugs of yak milk preserved via traditional methods using salt and lime juice, these are sold as "Himalayan Chews" or "Churpi Chews." The label claims that these are cheeses traditionally eaten by the people of the Himalayas.
The cheese softens after a little while, allowing the dog to scrape it off and eat it... and make a fair amount of mess, if your foster pup's not a super neat eater. Dogs seem to value these more than just about any other chew toy I've tried (although some prefer rawhides) and may quarrel over churpis if they're resource guard-y about food. They don't last quite as long as antlers, but on the other hand I've never heard about them cracking teeth, either. They are a good option for overnight crate chews, since they last a while and the crate contains the mess.
Hooves: Dried cow hooves (possibly also other animals' hooves, although I am only aware of cow hooves being sold commercially). I have no experience with these. I heard too many stories about dogs breaking teeth on hard hooves and cutting open their gums on sharp ones and decided it wasn't worth the risk. Use your own discretion, of course, but I've honestly never heard anything good about using hooves as chew toys.
Nylon and Polyurethane Bones: Nylabone, Hartz, and other companies make these in a variety of shapes, sizes, and textures. There are varying durability levels so that you can choose a bone tailored to your dog's chewing intensity.
I feel a little guilty sometimes because Whole Dog Journal doesn't like these fake plastic bones, but I do. They are all made of chemicals and synthetic stuff. Some dogs will reject them because they probably smell and taste artificial, being made entirely of chemicals and synthetic stuff. But none of my fosters has ever turned down a fake bone and they last a long time and they don't break teeth, and generally they're not valuable enough to fight over, so I can leave them lying around the house all the time.
So I guess you have to decide for yourself whether you're okay with your dog ingesting tiny scraps of plastic and choose accordingly.
Real Bones: Actual bones should be divided into two groups: baked bones (this includes the clean white sterilized ones) and raw or lightly boiled meaty bones (made at home from raw bones; as far as I know, no one sells boiled bones commercially). These are all bones from large mammals -- pigs, cows, bison and so forth.
Baked bones are too hard for your dog to chew safely, pose some risk of splintering when broken, and, in my opinion, are not safe to give to your dog. Raw or boiled bones are safer but should be further subdivided into marrow bones and knuckle bones. Marrow bones can still be too hard for vigorous chewers and should be taken away once the marrow's gone (although you can then re-stuff them with other fillings and use them like Kongs); knuckle bones can be eaten until they get small enough to pose a choking hazard. Both types last a moderate amount of time, make a pretty big mess (which may be a raw meat mess you don't want in your house), and are valuable enough to spark fights between resource guarders. Therefore, they are best given when the dogs are separated by crates or other barriers, unless you are completely confident that your pups won't squabble.
Rawhides: Rolled and/or compressed pieces of animal skin (usually cowhide, sometimes pigskin) shaped into knots, braids, circles, and all kinds of designs themed around major holidays. These may be flavored, bleached, or dyed (or painted with titanium oxide to turn them a nice pearly white -- avoid those!).
The best treats are made from thick sheets of undyed, unflavored rawhide. I avoid sticks and gimmicky shapes made from compressed scraps, because most of these are dyed and many are artificially flavored. Additionally, you don't know what's in the binders -- companies are not legally required to disclose this information, and given what manufacturers are willing to shove into pet food, I do not have much confidence that these rawhide treats are safe. I would advise sticking with the single-sheet rolls, although they are more expensive.