If you're talking about the thick mesh gaskets between the silencer and cat, i've taken pipes off dozens of times and always reused them. Plus when i was going back and fourth between the tors and stocks when i first got the bike (trying to decide which i liked best) i swapped the gaskets from one pipe to the other 4-5 times. The seal at that point in the pipe is IMO not a big deal. As long as it's not falling apart to the point theres no enough to allow clamping the pipe to the cat solidly. As long as they aren't so ragged so that is an issue, the only other issue might be a bit of extra noise. So go ahead and reuse them as many times as you want unless they are literally falling apart.