HAZ is heat affected zone. I'd say in the case of my TIG welds, its about an inch and a quarter wide. You don't want to work the material outside of this area as there is no damage done to it. I did find myself going outside of it on the passenger side as poor panel sizing and overstretching gave me a fit.
Grind down any type of weld before planishing as you mentioned previously. I understand MIG weld is a little more brittle, so don't do as I do
, do as I say and try to not overstretch it.