Not even close to the same extent as with software buttons. The little you can do is usually not worthwhile. I'd even guess most ROMs don't even include much, if any hardware button customization.
You can always add software buttons to a phone that doesn't have those with custom roms. However you can't add hardware buttons with customs roms. Ergo phones with hardware buttons are the better option if you're into custom roms.
I'm no longer interested in custom roms and I haven't seen any good use of software button customization so far.