I am with Rookiemower, it depends on the application.
If you have something that takes alot of abuse and shock, you may want a grade 5, a grade 8 will be stronger, but will be more inclined to shear. Look at the bolts on plow rigs...grade 5. Look at the bolts used to mount big strong trailer hitches...grade 5. I don't think there will be much need for anything beside a 5 in a lawn type application.
Jay