I struggle with this question myself.
I do often feel inclined to give a little bit of a tip.
Here's why:
Take out does require a bit of service beyond just paying for the food.
Taking the order and bagging it all together.
Serving food on dishes is easier than bagging it up.
At a restaurant, their main business is dine in. So the take out portion is a little extra work for whoever they have doing that.
If a place is just takeout, then the service should be built into the cost and require no tip, as the takeout is the normal routine, not an extra thing above the norm.
Same with fast food. The whole job is getting food to you and is built in.
Tipping for takeout is one of those border line things. Can go either way.