Re: Communication Issues between young and elder, alike!
I use them in a limited way, to reinforce something I'm saying, so I might write "Thank you, that helps :) "
I don't rely on what are, after all, nothing more than a grouping of punctuation marks alone to communicate the inflexion of my voice or my facial expression, so I wouldn't use them in an attempt to turn a sarcastic comment into a light-hearted one, unless I knew the person I was addressing very, very well - and I don't know anybody on here sufficiently.
When they are used in that way, I find it a bit arrogant (for want of a better word) because the writer is expecting the reader to make the effort to understand what s/he means, whereas I think the onus is on the writer to make him/herself clear. Having a joke and a laugh can often be done far better orally than in writing. My personal rule of thumb: if it reads snidey, sarky or mean without the emoticon, re-write it until it doesn't.
Edited to answer your last question: yes it does matter, by and large, if you could have said it in a way that wasn't hurtful. The only exception that I can think of off the top of my head is if the other person took offence because they were determined to do so when nobody else would have.