Do you have to tip when you are ordering to go?My grandfather taught me the art of tipping, and the general rule is "when in doubt, tip."
However, as far as takeout, traditionally, no you didn't tip. However, its starting to be expected at some places. The reasons are largely historical that tips in the U.S. are expected to be a gratuity to make up for the money that the employer doesn't pay as a performance differential for services rendered personally. The flipside of that is that now anybody who is underpaid expects some tips, because they often have to move between tipped and untipped jobs to make ends meet.
Tipping, however, should be used tactically. If they know you, its a small community, or you represent someone else, you should tip early and often. For example, if you expect to be eating there a lot, tip. If you work for a company and its a company order, you don't want your company to look like cheapskates, tip. Recently there was a YouTube video on the "I represent a company" tip fail. The effect cost them a lot more than a couple of bucks.