I've never heard of a rule. Maybe with booze or smokes? Maybe some states or cities have some weird law like NJ does about gas.

I've bagged my own groceries before when it was just the cashier or other baggers were busy or something. Also there is the self checkout. If anyone were to complain I'd just joke, "don't worry, I was a professional in a previous life". I did actually work retail and bagged stuff while being a cashier like a decade ago.

They might think you're odd or think it's weird if you rebag things like they did it wrong. Maybe be annoyed if you were slow. I don't think too many employees would actually care or get insulted. No one under management gets paid enough to care.