The WoW executable and associated data file are pretty interesting from an installation standpoint. When I first started boxing, I ran five separate instances all from the exact same WoW directory (granted, this made add-on management a bit more difficult than it really needs to be). So, you could probably leave your installation as-is with no problems. Once I started to get sick of minor annoyances (add-on management, being able to login faster, etc.) I split things into five separate directories with the bulky data files symlinked (WoW's data files are something like 13GB currently, so you don't want to have to make multiple copies of all that, and it's not at all necessary; just search the forums for symlink).



Note: I'm running Vista x64, if you're on XP or Vista x32 symlinking may be somewhat different or you may need to use an entirely different command.