ATM Must mirror-back be flat?

[RICHARDBAL@aol.com]

Seasons Greetings to all,
Several weeks ago I chipped my 8" mirror after an hour and 15 min. of hogging
out with 60 grit.  After receiving much sage advice and encouragement from
this site I decided to flip the mirror blank and start again on the back side.

All is going well.  I am encouraged as a first-timer and have achieved my
target sagitta for an f 6 by the spit test.  I am just starting 220 grit.  No
more chips (knocking on  wood as I write).

My question:  do I need to re-flatten the back?  If I don't will I introduce
distortion in the mirror when I press my laps?  I.e. Can I achieve even
pressure across the disk when I weight it down to create the lap?

If I must grind it flat what should I use as a tool?

Many thanks in advance for your kind consideration.

Richard Ball