I've used a variety of methods, and I find that different ways are good for different effects.
The rendered cloud method is great for the more atlas/satellite styled maps, as you can get some nicely detailed coastlines. It's a lot easier and quicker than trying to dray all those tiny inlets and bits by hand and still keep them looking semi-random/natural
For hand drawn styled maps, I either draw them out by hand entirely from scratch, or start with the rendered clouds for the randomness and draw over the edges on another layer.
Personal preference there, of course.