No, once the spider cracks are repaired they should be good, if it's done right there shouldn't be any need to repair them in the future. Sort of like a broken bone, the broken spot is actually stronger once it heals, and it may break again, but almost never in that same spot, if it heals right.
The bumper might crack in a different area in the future, but that's the nature of the beast. BTW, from what I can tell, SMC (factory body material post 73) does not have this problem. My rear bumper (aftermarket fiberglass, 82 style) does.
These are spider cracks, for those who aren't familiar:
