As long as they still have their conical shape and no obvious damage to them reusing them isn't typically a problem. A common problem is people over-torquing the bolts when using them and flattening them out rendering them useless. Well, not useless, just effectively turning them into flat washers.
Edit: something I thought of is that sometimes directions say to flatten them then back off to a designated height for proper installation and this is fine as well. What ruins them is if they are flattened and kept that way as they eventually just stay flat and aren't really doing their job if flattened out.