I get you, but cashiers are trained to be impatient and never wait for me to pull my change purse out to round it up, even if I tell them there's more coming.

This is how it typically goes:

Cashier: Your total is $10.50. Me, handing them $20: Here, hang on to this, I've got some change. Cashier snatches the money, enters $20.00 in the machine and stuffs it in the drawer as I fish out the appropriate change. Me, as they are in the midst of gathering a large amount of change from the drawer: Here you go. Cashier, interrupted during their change counting, furrows their brows at me as if I just tried to pull a fast one on them.

It can't be a quick change scheme if you haven't given me my change yet. Just don't be in such a rush.