I fully agree that $20 is a lot of money, but look at it this way, get it wrong and it can cost 20 times that in repairs.
I cannot say that I have never re-used a copper gasket, because in some situations one just has to use what is available.
I am reasonably experienced and can say that if you do not get the heating very, very close to uniform, when you cool it the copper can have varing states of anneal.
You can have one side soft an mallualble, and the other side a little harder, this will cause the tensions to react differently.
Copper is very forgiving and usually heats pretty uniformly, but the inexperienced can often get it wrong.
Take the advice of a silly old fart, us the $20 and play safe, if you cant afford one, I will offer to pay for it, because I like to see things done correctly.