Hmm

If I can adjust an mp3 version I would.
Or use wma instead (and see how the end result turns out).

Panning (which affects phasing) is a real possibility.
So is the hi-freq content (especially when combined with phasing once compressed)

If it's a post for a forum thread I wouldn't bother and agree with Scott and ROG that I'd accept it or not post it .. but if it were something someone needed for their website/ video I'd rework it until it sounded right at the end result (format). Depends on how much it means to you.

A posted mp3 (or video) should not be assumed to be indicative of the original, and actually if I liked your MP3, then bought the CD I'd be disappointed if the CD didn't sound better than the MP3 (or m4a or whatever).

The audio for a TV commercial using a known song is usually not the same quality as the original. This is intentional for the use.

Same thing applies.
Compression is usually 'lossy' and sometimes you notice it more than others.
.. and sometimes you need to rework it.

I do not work here, but the benefits are still awesome
Make your sound your own!