When I come across things like "Hello everyone!" or "My English no good sorry!" or "Thank you for all you help!!!" I routinely edit them out. More than once, though, these edits have been rolled back not by the quærent, but by others with sufficient privileges. In one recent instance, the "thank you" was the only element restored!
My own opinion is that salutations and thank-you's are too chatty for our Q&A model, and that they distract from the content of the question. What interests me about this site (as opposed to, for instance, forum.wordreference.com or usingenglish.com) is the Q&A paradigm and the notion that we are helping to compile a reference site. The absence of conversational diversions, however well-intentioned or harmless they may be, is what can make ELL more than just another newsgroup redux. In that respect, I think we should endeavor not to be different than other SE sites.
I poked round and came across what seems to be the protean discussion of the issue at meta.SE: Should 'Hi', 'thanks', taglines, and salutations be removed from posts? . The conclusion there (I think) is that we (the SE we) prefer to do without such elements.
In How helpful is it to add "Hope this is helpful!" to an ELL answer?, Ben Kovitz and F⚡︎F between them seem to espouse the cut the phat line, and my own view pretty closely conforms to Ben's answer.
So: do we have a policy? Do we discourage salutations, thank-yous, and similar chatty bits? (Meta question: Is there even a useful antecedent for we in those two questions?) Since I wrote this question, I've learned that we do. Nevertheless, I'll leave this question in the corpus because the title specifically mentions thank-yous, and it may serve to direct later searchers to F⚡︎F's original.