I was just wondering if regular domestic flyers knew how often seat movements for trim purposes happen?

My main reason for asking is that I never choose rows one or two on a middle of the day domestic flight operated by an A321 having previously been moved from row 2 to a nasty middle seat at the rear of the plane.

I turned down the opportunity for seat 1C on the 12:35 GLA-LHR this week based on my fear of being shifted. Foolish or sensible? I guess I'll find out when I fly!