Never. I've never heard that this is customary. They're just a cashier ringing up your food. Do you tip the cashier at Taco Bell? Tipping wait staff is because they bring the food directly to your table, bringing and refilling drinks, (should) share with bussers for clearing your table and host/ess for seating you etc... To grab a bag or two of food off the counter behind them and ring it up isn't tip-worthy.
Often times, especially at non-fast food restaurants, they are not just grabbing a bag and handing it to you. Say you order a takeout from a steakhouse that is a steak, baked potato and salad. Do you think your salad dressing, butter and sour cream all just magically appear in little containers? Nope. The takeout person spent time putting that all together, so they did not just grab a bag and hand it to you. I am not saying to tip 20% on a takeout, but a few bucks is more than doable, IMO. Plus, in some places, the bartender is the takeout person (I never had to do it, but I sympathize with those who have had to), so they are taking time away from customers who will tip them to put your takeout together.
tips already have ballooned over the years (now 20% is essentially expected).
I agree with this. 15% is not a bad tip by any means, but a lot of servers nowadays are not happy unless they get 20%+. Granted, they probably want 20+ from the "good" tippers to counteract the bad tips, in the hopes that their average over the course of a shift comes out to 15% or more, but still, 15% is not a bad tip by any stretch of the imagination. Me, I never tip less than $5, even if I get a bill for only $15 (like if I am out and do a quick stop at Texas Roadhouse and get just a steak and ice water).