I have fostered a couple of shelties and a few stray mongrel pups. Like Chris, I took them in knowing I would not keep them. With the dogs that come through, I usually feel that they deserve a family all to themselves, not one, mine, with 2 dogs of my own and so many other commitments (I run a sheltie rescue so will get fosters now and then).
I find that if you've had shelties or other dog experience before, especially with rescues, fostering shelties won't be
that difficult. Fostering does take up time and energy and can test your patience. You may encounter things like fearfulness of people and things in general, and zero toilet training, especially with puppymill rescues, but with patience and love (and some reading!), you'll be able to help them. Some also come with health issues, so you'll have to be prepared to give medication regularly, and feed special diets. I assume you'll get some support from the rescue you volunteer with? The fact that you have nice dogs too, helps! All my fosters learn from my dogs!
The only fosters I've had issues with is those that have aggression problems, who react with their teeth when scared, or just pissed off

. Those require someone with more experience. But I personally only see that sort of behaviour from shelties that are given up by owners who basically don't interact with their dog, or tie them up in a corner most of their life

. I don't come across such shelties very often. Most are gentle and easy to manage, just scared and not trusting.
I learn new stuff with each foster that comes by. So long as you are committed in helping them overcome their problems and issues, so that they can start their new life, you will find a way to make it work and find solutions to problems you encounter.
It is extremely rewarding to foster a sheltie, watch her blossom then send her to her new family, knowing she will be leading the life she deserves with great people who love her and will care for her till her last day.
Extremely rewarding.

Someone who helped me foster a sheltie is now so into animal welfare and dogs that she has taken a distance learning class on dog behaviour and is working towards getting some kind of related degree! I guess the experience changes a person
