I used to work as a grocery store bagger and it honestly depended. I always said "yes" when somebody asked because I was more than happy to let somebody else do my job, but there were definitely instances in which they bagged things so badly that I had to take over and correct them. Believe it or not, there is a tried and true "method" for bagging (both paper and plastic) and there were some things people did that made it borderline dangerous for their health or just impractical. Think: putting cleaning chemicals with the meat, or putting heavy stuff on top of the bread.

I definitely also much preferred people asking compared to people walking over and just pushing me aside, which they did many times.

If you don't trust yourself to bag things "right" so that you don't end up with a mess in your car when you get home, then just let somebody who's trained do it for you. If you do trust yourself to do it, then at least ask politely.