She's just at the right age now. I know most say 8 weeks old but traditionally it is 6 months. The main reason many shelters, etc do the procedure earlier is because in the *old days* many adopted and promised to spay or neuter their pet but didn't.
Anytime is a good time now that she is six months old. Cats just like human females have periods. That means the blood will come out of her vagina. Also, if she gets ouside (if she is an indoor cat) the boy cats will definitely try to mate with her and she can get pregnant the first time.