I never used to tip on take-out, but now I regularly throw on 10%. Seems like a generous thing to do and it doesn't really bug me.
But if I don't feel like it for some reason, I also have zero guilt over hitting the skip button and leaving no tip whatsoever. Just because a tip is the default option doesn't mean you have to use it. I've been at a few places where the counter person hands over the machine and tells me to "just hit the skip button." Even they realize the idea of a tip for that kind of transaction is ludicrous.
But I agree that the standard options of 15%/20%/25% are pretty stupid, especially for takeout. I was pleasantly surprised last summer when we stopped for lunch at a St. Hubert in QC. Wasn't a proper restaurant, but kind of a fast-food hybrid place. When they presented the payment machine, default tip options were 7%/10%/12% IIRC. That seems like a smarter way to go about it for places that aren't proper restaurants.