Sure, and I do always strive to be nice to anyone I have to interact with whether they're cashiers or otherwise. Social anxiety doesn't tend to be a rational thing though, so the fact that consequences aren't unlikely doesn't really make it go away. I know that nothing bad is going to happen from a minor social interaction, and yet that does absolutely nothing to make me dread it less if I'm happening to be in an anxious state. A lot of mental health things work similarly to this; for example, having depression doesn't just mean you're sad because things aren't going well, but you can be despondent despite everything going perfectly fine in your life. If you can't imagine having feelings like this that conflict with what you actually know about a situation, then you're one of the lucky ones, but you probably do know someone who would know exactly what I'm talking about.