WAVs can contain metadata (although there is no official list of tags). What rekordbox writes to the footer (lead-out) section of the track is the additional information you enter into the fields of the database. Here's a sample of what it wrote when I added comments, the genre and an album to a track that already contained information for the title and artist (which was contained in the header):
ID3 .ID3$COMMXXXThese are my commentsTCONDanceTALBSuper Funky Album
This will not corrupt your WAV file as it is lead-in / lead-out data. Mixed-in-Key won't damage your files either. All this does is provide data that rekordbox could call from directly off the file rather than needing you to input it solely to the database.
Although, interestingly, I removed and re-added my sample track and all but the comment remained... it could be due to the fact the track already had comment data contained in the header and thus prioritized that information rather than the data I added after. This is a perfect example of the problem with there not being any standards for WAV file metadata. Consider using AIFFs where possible!