Do we want smiley faces in questions and answers on this site?
In general, one could say that these are distractions, not unlike intentional misspellings or extraneous punctuation marks (especially of the exclamation variety). But I don't think they're anything of a threat that requires hunting them down or making any sort of harsh policy on.
Rather, I'd recommend treating them in a similar vein as one treats signatures or taglines or "thanks" - if you're editing the post for other reasons, then you should look into removing those. Or, in other words, don't go out of your way to handle these. Unless there's an incredibly widespread problem with smileys in posts, it probably isn't a major concern to work on them other than organically through your normal browse day.
You can probably afford to be a lot more lenient on a Meta site as long as a person isn't abusively decorating their entire post with them. Likewise, if it's part of an intentional and colorful point that is contributory towards the answer without detracting from it, then it is also not all that necessary to remove even when you're editing other parts.