Pro Tools 3.0 and later gives you an option to select relative or absolute position for existing regions when you change the Session Start Time (Time Code window in Pro Tools 3.x. Session Setup window in Pro Tools 4.x). Choosing Absolute would keep the regions at their same SMPTE locations. This would be preferable if locking PT to video, where the regions have to maintain specific SMPTE locations. Choosing Relative would keep the regions the same distance from the session start. This would be acceptable when working without Time Code, since the region locations have no time relation to an external source.