I filter it with a plastic, almost see-thru fuel filter I picked up at Autozone for like $5. Works great.
Micah,
I think the main reason to do this is to stop letting a polluted air charge into the IC which can, and most likely will, ruin it's heat transfer capability. I know the first time I cleaned my IC with solvent, a lot of build-up came out besides the oil sitting in the bottom IC tube.
Another good reason to do this is that any oil vapor in the air charge reduces the amount of air that the blower is moving. It may be less than 1cfm per revolution, but the poor M90 needs all the help it can get.
Finally, any A/F ratio changes that it may cause should easily be correctable with a tune.