if they are undamaged, they should be OK to re-use. FWIW new chainrings often 'settle' in the first period of use. Shimano and other manufacturers often overtighten the bolts in new chainsets so that they are not so likely to back out and loosen etc but IME if you grease the threads and use the correct torque on a new chainring, it isn't unusual for the joint to settle in use and for the bolt to need retightening.
Once a chainring has settled, it doesn't seem to be likely to settle a second time, not on the same spider. I think the settling stems from there being microscopic burrs on the parts, and that overtightening the bolts in the factory helps to squash them. However this (and that they seem to use no grease on them) does nothing for your chances of getting the bolts undone months or years later. Thus my habit is undo new chainring bolts on new chainsets (with difficulty, they are usually v. tight), to grease them (on the threads only, both halves, the rest of the part should be scrupulously clean) and then reassemble.
Presumably because such an assembly has been overtightened once already, the joints seem less likely to settle and need retightening. Needless to say it is an excellent idea to check them after a hundred miles or so anyway.
I find that if you use decent grease as specified, bolts so treated come apart again quite easily even years later, no peg spanner required.
cheers