It isn't really the norm here, but I always do for humans if I have a full cart or something to complicate the cheek out process. When I worked at Wal-Mart and I wanted to grab a snack on break and had to buy one, I would actively ask people.  Humans usually had no objection and some would even offer me to go ahead without me asking. If a ham said yes,  it was with a mean look look or tone of voice.