From a designers point of view - compress if you really need to, but as Fris said, keep a copy of the original if there's a chance that you or somebody else is going to make any edits later on.
If I receive a compressed CSS file, I just quickly re-format it with an online tool such as
http://www.lonniebest.com/FormatCSS/, so removing spaces etc. isn't a problem but it's a real pain when you have a massive CSS file and absolutely no comments
So if you're lucky enough to have a commented CSS file, keep a copy of it
