G'day Julie,
The kudos system exists to allow people to "reward" good posts. As such, I would say, don't kudos someone in return unless they have made what you consider to be a worthwhile post.
Now, WHAT constitutes a worthwhile post? Well, for you, I don't know. For me, it's a post that gives value to others (more than most), turns on a light for me, or makes me laugh more than I have for a while (and there are probably more reasons which escape me for now...).
We can't kudos everyone for every post (or we wouldn't have time left to read them all). So, I think, save your kudos for those "special" posts - whichever are special to you.
If you wish, by all means use a PM to say "Thanks" - but, for me, when I give kudos, I certainly don't expect a PM in return. I'm just giving as I see fit, and am not looking for ANYTHING in return.
Others may well see things differently, (as might you), and I'd be interested to read how others see "kudos" as well.
Good question, Julie - I hope you get your answer,
Regards,