When my 5D was new I had the oily streak problem 2 or 3 times. I cleaned it with the Visible Dust sensor cleaning solution and swab kit and washed the brushes with their brush washing tablets dissolved in water.
After a time it settled down and now it just gets dusty!
In my experince it's neither better nor worse than the 20D or 10D for picking up dust. I don't avoid changing lenses in dusty conditions (I'd prefer to get the shot) and it does get a dusty sensor from time to time.
When it does I simply clean it with the full size brush (not the Arctic Butterfly). Far from not touching the mirror box walls, I clean them from time to time with a smaller brush, but I never use this one on the sensor. I believe keeping the mirror box clean will help to keep the sensor clean.
It varies, but I'd probably average a sensor clean about every 2,000-3,000 frames. I do work in some quite dusty situations.
Regards
Frank