Throat erosion question

At the start of 2009, I had a couple of 13.5 tw barrels chambered for my Panda. having tested and found the magic seating spot for boattail bullets at 7thou off the lands, I had a good measure of success shooting point blank benchrest locally.

After about 400 rounds, I found inconsistencies in group pattern (larger than accepeted group sizes).

My question would be: is throat erosion directly relevant to chamber pressures generated ? and; as such, how often should you adjust seating depth ?