A good number of the GMs who post here have taken to making their own 3-ring binders with the most frequently used or even the rarely used/difficult to find charts and tables. A photocopy and some scissors and tape and you can piece together a set of handy, somewhat related charts onto a single 8.5"x11" sheet of paper, then pop that into your binder.
I have a binder with all the new weapon tables and crit tables, MM charts.
One binder with everything needed for PC creation; races, background options, stat gain table, skills from the Companions.
One binder for GM goodies like treasures, poisons, herbs, creatures, etc.
I didn't photocopy every single page from every single Companion, but I was able to write some notes on my copies to refer back to the full blown description in "RMC-III pg. 36-38", if I needed more info for that topic.
It will be difficult to do with RMU right now since it's still in development and the next Beta is coming out soon. If it does have additional Companions like the previous incarnations had, then your RMU Cheat Sheet is going to grow.
What makes RM so great is the variety and the options, sadly, that makes it nigh impossible to have just "one cheat sheet." Unless you used a roll of wallpaper! ... and I wouldn't trade that for all the Mithril in Shadoworld