Just posted in DAP is a discussion of 6-bit lens coding for the Leica M9, and step-by-step how-to instructions for coding a lens with the Match Technical Coder Kit. I’ve also posted a free mini review of the Coder Kit.
Because of the way the Leica M9 uses the 6-bit lens code to modify the RAW file, my experience is that all M lenses should be coded— more than once I’ve had the wrong software corrections applied by failing to set the right code manually (using the M9 menu system). That effect is not undoable, so you must better get it right eg code the lens.
The Match Technical Coder Kit is a no-brainer for Leica M shooters with any uncoded lenses. You might also check out their ergonomic offerings for the M9/M8.