the only thing one has to worry about if they have the PHONE side on, not the DATA of course, is that calls that come in to voicemail for example, one gets charged even if the phone isn't answered. If you get a lot of calls and also VM's, then this can be $$ after a few days even if you haven't even answered the phone. Historically, AT&T charged for a round turn, inbound call, VM alert, confirmation, so it would be 2x, the per minute charge for whatever country one is in.

I simply turn off my VM when I leave the USA (call to 611) and then answer as needed, or I call people back with Skype or Vonage