Is there a reason to not grate cheese ahead of time? Suppose I know that I'm going to eventually end up grating all of my cheese. Is there a reason to not just toss it all into a blender and store it for later? To be more concrete, I'm particularly interested in the answer as it pertains to Parmigiano-Reggiano and Pecorino Romano, both the bonafide sorts and any variations that can be sold under the same name in the United States.
You should only grate cheese as needed, particularly the cheeses you mention.  When you grate cheese you create more surface area.  That is more surface exposed to air and oxidation, which will degrade the flavor and aroma of the cheese.  It is also more prone to drying out further degrading the quality.   The cheeses you mention are best when grated for immediate use, but stored in chunks, wrapped and refrigerated.