You SHOULD replace the nuts, mostly because the price of failure--while unlikely--is very high.
I replace them, usually. I suspect that most people do not.
These nuts are specially threaded...kinda like a triangle and they tap onto the bolt and do not back out. This special threading is degraded or lost upon installation...which is why it shouldn't be reused. You can't find these at a hardware store.
You can always install the old ones and get the new ones later. Depends on how hard you drive, how often you check over your car and whatever. And how difficult replacing the bolt will be...
If it were _me_, I would do the job, order the nuts and then inspect/install on the nuts at a later date. It is very easy to rip the axle boot, so be careful.
Drew