Being fairly new here, I can only give my own opinion on this. While saying thanks to a lot of people might fill up the what's new section, the oldest thing currently in there is 5 hours and 45 minutes old. Thank you messages, even if there's a lot of them, will get drowned out of there without causing annoyance, I'd say. Plus, saying thanks is part of the forum, so it's valid activity in my book. On top of that, saying thanks is part of the general friendliness and supportiveness that keeps me around a lot. Nobody here's rude, and I've had nothing but friendly and constructive feedback on my work so far.

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!