I have a K&N filter and do not have any problems with the mass air flow
sensor.  If you wash the engine you can risk poppint the electronics
inside the sensor.  If you like to wash your engine stay away from the
sensor and the throttle body.  I had a 85 corvette and blew the mass air
flow sensor by washing the engine off.  I got a little too close to the
MAS when washing and it was a little over three hundred bucks to replace
it.  But, I have K&N air fliters in all three Nissans and the 92
Corvette and have no problems with MAS.