Everyone sayin don't tip on carryout has obviously never worked in a restaurant.
They're doing the same job, and it's more inconvenient to them.
They pause what they're doing to take your carryout order, write the order and then box it, put all the silverware, napkins, etc. in and have it ready foryou. You should tip them the same or close to what you would tip them if they waited on you.