Thanks Pawel, Pat, and Scottrc.
Getting a sharp demarcation between colors isn't that hard on a smooth surface. If you look at the edges of your roll of tape, you'll see that it's picked up pieces of dust and debris from being exposed to air, paint, etc. Before I put it on the model, I put the piece of tape I'm going to use on a piece of tile, take a straight edge and a knife, and cut the edge with the dust bunnies off. This gives you a nice clean edge, and then I burnish the heck out of it to make sure no paint seeps underneath. I try to remove the tape after the paint has set so that it doesn't form a ridge as it dries. I usually do this within a couple of hours of painting, depending on the type of paint used.