Are you reducing your power on descents or maintaining cruise power. If you are at 65% or below, it probably does not make that much difference. If the engine starts ot run rough, richen it until it stops, and maybe a little more.
According to Deakin (Pelican Perch on AVWeb) there is no need to rush richening the mixtrue on descent, and if you are at 65% or less power setting, then you will not hurt the engine either way. Too rich or too lean you can cause theengine to quit, but correcting the mixture will (should) cause it to resatrt spontaneously.
A big plus for the ARNAV EngineView is that is shows the % power.