ATM Square mirrors?

["Andersen, Clark R." <clanders@utmb.edu>]

I have not yet gotten around to an attempt at grinding my own mirror, so
this is likely a somewhat naive question, but I'm curious as to whether
there's any good reason not to parabolize a square blank of pyrex vs. a
round one.  I know that the image produced should be the same, and it seems
to me that it ought to be cheaper and less wasteful to cut square blanks.
Would the polishing be more difficult?