I would agree with budman3, I tape based on vehicle and how much plastic it has. Another reason I don't like vehicles with a bunch of plastic around them...any who. Taping is a bit of a pain but when you’re just starting to detail vehicles it's a good idea, once you get good at it you can probably do with out it but it doesn’t hurt to tape things off.
budman3, when you say you use products that don't stain which products are you referring to? Reason I ask is because I was under the impression that all polishing/compound products would leave that white residue? But you are correct dressing the trim works great.