I think you can never be too polite.

However, I personally don't feel it's necessary to have a four or five-response thank you session if I 'like' a post or give someone rep. I do it because I feel the post deserved it, and hopefully you'll see something of mine that you feel deserves the same, end of story, that's all. Everyone here is pretty darn nice, in my book, so if I give someone rep and they don't thank me for it, it doesn't bother me a bit. I just assume they'd rather make a new map than trade thank-you PMs for a week. Just as I assume people don't get offended if I don't respond to a thank you PM, etc.

Also this:
In short, don't overthink it! Say what you want, there's plenty of stuff happening to refresh the 'what's new' section in a negligible amount of time!