One of our friends (Miss Represent, UK DnB dj) brought this subject up of MP3 mixes and rendering to Wav or to MP3 - she found that the rendered MP3 mix sounded worse that the WAV would, purely down to the double file compression. If you're doing just the one pass then fine but if you plan on using in a mix for some reason as is and then bounce down to WAV then MP3, you're constantly degrading the signal and raising noise level due to the compression algorithm in the MP3 codecs.
Why? because surprisingly if you rip a compressed format (think Youtube included) such as a Youtube MP4 to MP3 conversion, you're actually degrading the MP3 from the off. record to WAV directly via loopback software.