When I shop at a smaller store, like Walgreens or CVS, there is usually only one cashier. At larger stores, like Kroger, Target and Walgreens, you usually have several cashiers to choose from. Choose wisely!
Call it "Cashier Profiling," but I carefully survey the cashiers to see who I think will give me the least hassle. Not because I'm using the coupons fraudulently, because I am trying to make the process as painless as possible for both of us. My experience has been that middle-aged women are the worst. No offense to all the middle-aged women out there. I'm one of you!
I have had the least problems with young, under 25 cashiers - usually male. If there's one of those I will pick that lane every time. Sometimes I will also seek out a manager that might be on register. They tend to be more aware of coupon policies (but not always) and if there is a problem, they can make the decisions themselves.
If you do get a bad cashier try the "if at first you don't succeed, try, try again" motto. I have been known to give up on certain items that are not ringing correctly and go back at another time and they go through with no problem. Since I buy multiples of items, sometimes I use the "yesterday they let me..." line with some success.
Like I've said previously - I use my coupons only in legitimate ways so usually I'm pretty confident that I'm right. I am generally not an arguer so if my first attempt fails I usually do not purchase the item and try again at another time. You'd be amazed at what difference a cashier makes!