AT&T doesn't really bust people for tethering and going over the soft 5 GB cap. Maybe if they go ridiculously over. Pretty much just don't use it to download a bunch of movies and you should be fine. But if you call tech support for something about your phone and they figure out you've messed with it to enable tethering, the only fix they'll give you is to put it back to stock config, i.e. without tethering.

Unless of course you're outside the US and on a carrier that doesn't disable tethering like AT&T does...