Another thing I've noticed with cashiers in general is that niceness doesn't completely pay off. Those who are nice are sought out by customers who love being rung up by them. So they get more customers during their shifts in comparison to those who are not so nice. Ironically the later have few customers b/c many attempt to avoid them. (I happen to have some really nice cashiers by me. Is there some way I could show my appreciation? Tell the manager? Give a card?) //end of tangent//
I wouldn't send a card. But you might tell the manager, in person or in a letter, how nice those cashiers are and how they make shopping in the store a pleasure. That would be a very nice thing you could do for them. They might even get a bonus out it -- at the very least, the manager would give them a pat on the back and remember it.
If you also want to tell them personally what a pleasure it is to do business with them, I think that's fine, but I would keep it short and sweet so that (a) you don't hold up their line , and (b) so they don't think you're trying to ask them on a date. I'm laughing, but I'm serious -- especially if they are female, they've probably had it happen a few times! You don't want your very nice gesture to be misinterpreted. A simple "you're so nice you make it a pleasure to shop here," along with a smile, does the job perfectly without going overboard, IMO.
I have done this at least once -- there is a very funny, friendly guy at my local grocery store and it always makes my day when he rings up my stuff. And I've seen him handle difficult customers -- he's so good at smoothing them down. I did tell the manager how great he was.