I think there are some realities you really have to ask yourself before you open up your home and heart to fostering animals - one of the most important being, will you be putting anyone already living there at risk by doing so, particularly children?
Another being what type of experience will you be able to offer to the unfortunate soul coming into your home? Most animals will already be coming from difficult, sometimes horrific, situations so the experience you offer should be one of a welcoming, safe haven where they can readjust and recompose enough to develop stability so they can find there forever home. (The last thing you want being learning bad habits from your home.)
I wouldn't want to put my family/grandchildren at risk if we ended up with a dog that bites.
We currently have a little terrier mix (7 months old) who has been a hyper beast
These, IMO, are things to be seriously considered. And please don't take offense, they are reasons similar to why I myself am not fostering at this point.
In fostering German Shepherds (my favored breed), there are a lot of rules put in place for safety. Age rules govern not only children living in your home, but
children visiting your home regularly. I
have had young children in my home regularly. I also have a dog that doesn't love other dogs. Then there's the issue of physical space - a foster dog needs a space of his own. In our case, we also need to have a 6ft fence surrounding the yard that's in good repair. We are also required to have solid experience with the breed, especially for those dogs who've had little to no socialization or training or otherwise have special needs. (And there are other requirements as well.)
So really, it's kind of a tall order. And I do not fault the breed rescues because their rules are put in place for reasons based on their experiences in the past. I also know from my own experiences that I want to have these rules in place as well because I know they make sense and they're for the good of all involved. Now granted, German Shepherd rescues may have to be more stringent than those of most breeds, but I guess my point is that there is always a lot to consider. For some, it may seem silly; but for me, I prefer it this way. I will be fostering at some point, but the timing and circumstances will have to be right. YMMV
P.S. Remember that for those whose circumstances aren't right for fostering, there are lots of other ways to help that don't involve actually taking a dog into your home. Donations are always needed. Most rescues have a Wish List. Dog walking, cleaning cages, socializing animals, taking photos for websites, donating old blankets, towels and other items, etc., are some of the other ways people can help.