As I see it, karma exists more at the level of large groups, or society as a whole, rather than the individual level. E.g. letting somebody in in traffic may not do you any good. But if everybody does it, then it will. i.e. if we all behave nicely towards each other, life is better; if we all behave horribly towards each other, life is worse.
Calling somebody a pudding under your breath isn't going to harm you - unless they hear you ... :) In fact, I think letting off steam is a good thing.