I haven't ever been moved or even seen anyone been moved for trim on a domestic yet. My domestic experience is split between MAN services (A321/320/319) and LBA servies (A319). Even on lightly loaded (<30) I haven't seen any moving of passengers. I have occassionally seen some odd seat blocking prior to OLCI (me in 1A and all seats blocked up to row 6 - only a few were actually occupied) so I wonder if that was some attempt to manage passenger distribution.