
It is a myth that it will do a bitch good to have a litter.
You should only let her have a littter if it is something you want to do taking all the risks and responsibilities into account. If she is just a pet I would not bother, as there are plenty of experienced, as well as not so good breeders breeding staffies.
Spaying is not without side effects, and thre is no reason why a well manged entire bitch should ever have unwanted puppies, but I would spoay her if you wish once she has fully matured, preferably mid way between seasons.
just my humble opinion :D