Anymore I mostly upload wave files and let the site convert them according to its specification. The only times I make mp3s myself is for emailing or if some website requires it.
I export a .wav file from Reaper, load it into a new project and apply dynamics and EQ to it, then export it to MP3 with the LAME encoder. My computer tends to choke up if I try to apply effects on the master bus of a mix, so I pretty much have to do it this way anyways. But I like that I end up with a .wav file that has plenty of headroom if I were to ever send anything off to be mastered.
My question is why convert to MP3 at all? Is the reduced file size the only reason to convert?