I have two assumptions about this, first it only needs to be done if you change the placement of the MAF or the diameter of the housing and two, if you make so much power you outflow the stock maf scaling.
My friends trans am that im tuning just has a LS6 intake, ported TB, sloppy stage 2, ls3 injectors, and E30. I have not rescaled the MAF because I did not think it would be necessary but now I'm seeing ridiculous MAF lb/min #s like 60lb/min on a NA LS1. The car drives great, idles great, and pulls hard, he and I really have no concerns with the tune but that could be because I'm not seeing something some of you might.
I'm sure everyone has an opinion on MAF lb/min translating to HP and I have my own thats based off half a dozen cars. When the MAF is scaled correctly it can inform your HP assumption pretty accurately. Thats just been my experience so when I saw a MAF lb i really did not expect to see it got me thinking if I have done something wrong here.