Im at about 50/50 in using these header gaskets a second time.
When you get then, new, they are about 1/4 in thick. Im probably exaggerating, but very thick.
When you tighten the header, they smash to about1/2 of what they were. In that respect, time means very little in how they may seal again.
Odds are, you will be ok using them again. However, if even one leaks, you will need to remove the entire system to replace it.
I think you can get a full set, on ebay, for @ 12.00-15.00. Much less than the time you would need to remove your new system and replace them.
Again, real good chance you can get by, but I always change mine just because if the pita factor in replacign them if something goes wrong.
If they leak, your bike will always crack and pop, no matter what you do.
For best results, replace them and tighten. After a few hundred miles be sure and tighten them, again.