I'm for talking it over with the GA ahead of time. However, be aware that it may not turn out the way that you anticipate. A couple of weeks ago, I told the GA that I didn't want the bulkhead (only F seat left) on a 2/3 full flight and that I'd be happy with a row to myself in EC -- there was only one other person in the entire EC section. A few minutes before push back, GA came on the plane and asked me if I wanted 2D or 3B since there were no shows. As I settled into 3B the FA told E- pax that they could change to any seat, including EC. Just about every seat in EC, including the row I had vacated, got filled in minutes.