well encoded a + is %2B. No reason not to use dashes instead.
edit... let me rephrase that a bit... in order for the symbol to be proper in the url it needs to be encoded... in theory and from what I know G indexes a + like a - and if you're using +'s for rewrites in a dynamic string that could screw it up?
I just don't trust it... you need to have clean urls - if you're questioning it so much use -'s... they're safe w/o a doubt
Google changed that, to a degree, a few years ago. Today they can pull out a keyword from the URL even if it's only separated by an underscore. However, the value of any keyword in the file-path is quite negligible, so it's not worth the many challenges of changing existing URLs.
So the way Google handles underscores is better today, but dashes are still smoother sailing. Reference: Dashes versus Underscores [mattcutts.com]