Here is some good stuff on the MAF screen, and the calibration of the MAF:
Regarding recalibrating the MAF:
-Rara-
All that said, often times you can be fine without the screen, and occasionally you can't. If you do have uneven flow across the MAF, sometimes "clocking" it can help (clocking means rotating the entire MAF about its centerline) to put the sample tube in a better part of the airflow.
Regarding the screen...
-DemonSVT-
Stock MAF - Leave it in.
Aftermarket MAF - Leave it out.
Threads:
thread1 thread2 
) and Homemade Heatshield *
