I would definitely recommend t-moldings at every doorway. You can usually skip them in smaller closets, just be sure to undercut your jambs and cut the laminate so it can move freely under them. If you have any large rooms, (over 20 feet in any direction)double check your manufacturer's specs. Some makers recommend expansion joints with t-molding if the room is over 20 odd feet. If you're a foot or two over what the manufacturer recommends, I would say you could probably skip it, but if your room is 30 or 40 feet long, do what the manufacturer says. If you do choose to leave out the t-moldings in the doorways, make absolutely sure the laminate can move at least a 1/4 inch in every direction, at all obstructions in the entire installation. This means every wall, threshold, door jamb, closet track, bifold closet bracket, fireplace hearth...you get the idea.