Ah yeah, that's a lesson @amortelito taught me in not too long ago. (Domo arigato gozaimasu sensei!) 
There is actually the possibility in your settings to forbid comments / post on your wall from "strangers" alltogether. So you could excuse yourself with "oh sorry, you didn't deny me so I thought".
Edit and correction (April 6th) : There actually is not? I only noticed people I couldn't post on their walls. I assumed without checking first if there actually was a button of some sort to do that. Apparently there is not and this might be a different situation. Sorry for the misconception I may have caused ...
I think for myself (now) that thanking the reader for actually reading isn't a bad habbit at all. I am thanking the persons who are making my creation possible after all! I may have the ideas, but the drive and HIGH of people actually reading it concludes to me to gratitude.
I of course have my card, but I also always make an individual text when I thank them. Not just "thank you" but "Thank you [insert Name]." Like "Thank you, Mal, for making this thread". ^^