Usually I let it go. But I have had a cashier (or server) that has been so rude, I didn't want to return to that store/restaurant. In those instances, I usually look to see if there is a comment box, or an e-mail on the website for complaints.
I worked in retail and as a waitress while I was in college and can appreciate that some days you just don't want to be working BUT there is a big difference between not getting good service and having someone be rude without reason - especially as a paying customer.
I'd let it go, too. Unless they were snappy to begin with and crushed my stuff or mishandled things or their attitude was really bad. Then I'd step up and be vocal about it.
If you get a rude cashier while checking out what would you do?
Food Frenzy