weird thing that this is enabled by default for iTunes and iPhoto, essentially making a copy of all your media - because it is never stored where iTunes library is. With SSD drives being standard for all macs, space is luxury.
In my limited experience (friends/co-workers/clients) when this gets turned off, the library gets screwed up. Eventually media will get renamed or moved which will cause iTunes to lose track of it. Of course, if you are well organized and disciplined with your file management, that wouldn't be a problem.
iTunes allows you to change the location of the entire media folder. That would be my first choice rather than turn off that feature.
In either case, if you need access to the NAS for your media while you are away from your network, maybe a VPN would solve that problem?
-KeithP