When I have the choice I try not to go to supermarkets at busy times of day, but always have to use the checkouts with the buggy as it is hard enough for me to unload things onto the shelf in the first place. But my way is that I go round and get all I require and then, before I go to the checkout, stop the buggy in a quiet area, look at the things in the front and move the heavier stuff in front to be loaded first, then I always carry two or three different bags, so that the sturdy one is for the heavy stuff etc., and it is better for me to have 3 or even 4 bags with a small amount in than have a very heavy bag to lift out at home, so then everything is in the right order, and I get my loyalty card out at the ready and go to the checkout. Give that first and start loading stuff on and when they give me my card back I finish loading and move forward and get out my card to pay as they are putting stuff through. Usually I ask if they would please load things into the bags for me and as each bag is filled I put it into the trolley while they are doing the rest of the stuff. Then when they finish I pay with my card , get my receipt and am off. But you do see such a mixture of attitude.
The other day I watched a man probably in his 50's stand and watch the person putting stuff through without even getting his card out ready, and then asking right at the end after she had totted up the bill for a bag. You could shake them when they are like that couldnt you. My thing I like to do in Tesco, is get my blue tokens - which I think is a very good scheme and supports lots of local charities, which I approve of, but I get my tokens and if I see a mother struggling to pack stuff and deal with children, I ask them if they would like to put my blue tokens in the boxes if their mother agrees. It is often appreciated as they enjoy posting them and the mother gets those few minutes to get sorted out .