You'll probably find that in the manual (or on the back of the sun visor) it says something along the lines of 'don't lean the mixture at more than 75% power'. You'll also probably find that a normally aspirated piston engine won't develop 75% power at that height so you'll be OK to leave the mixture lean (it will richen as you climb). The problem could arise when you descend if you forget to richen the mixture before adding power on levelling off, as lower down the same mixture setting will be too lean and high power settings could cause nasties.