and to followup on what nicholas said.... there is a fair amount of horizontal "tolerance" even when Snap is off so that (in my experience at least) I rarely make an accidental horizontal shift when dragging vertically, either from track to track or within a FIPM track. I'm still a little paranoid about it, but in practice it's not really a problem. (It's nice that Reaper will show the item's position when it's clicked on, so you can watch the ticks and confirm you're staying vertical.) Semi-related to this, in case anyone's not aware of it, when Snap is on, Shift will override. (I wish the opposite was also true!)