I'm fostering to adopt right now.
The shelter I'm fostering for usually does let people foster to adopt. I don't know why rescues would frown on it.
I won't take in a dog (with the exception of pups under 6 months) unless I can take him/her home for a while without commitment, to make sure the dog will get along with mine. Sometimes it takes a couple weeks for a dog's true color's to emerge, and I don't want to adopt a dog right off the bat, only to see a dog-aggression issue or something pop up as the dog settles in.