There are still a handful of PAL games that don't support NTSC and they will require the Xbox to be in PAL mode for them to work, luckily the list is small.
The problem you may encounter, is if you decide to import an NTSC game to play on Live and that disc has region coding. Changing to NTSC with enigmah does not change the region of your Xbox, and if the coding on the NTSC disc conflicts with your region, you won't be able to play that game on Live.