With an all metal gasket like the copper ones I can't think of any disadvantages of using the copper spray as well. It would have to be a pretty bad fit to leak after several coats of that stuff.

I have also heard of people having to shorten their bolts because of crud in the bottom of the holes and or height removed from the deck.

That is another nice thing about the copper ones, after removal you can see if the gasket was flattened equally all the way around.

54 3100 with 235
62 flatbed dump C60 with 261