To add on a bit more...
If you have a synth, chances are it has a patch called 'percussion' or 'drums'. Every note is associated with a different percussive instrument ie, you'll have toms, woodblocks of different pitches, snares of different pitches, symbols, gong, all that stuff. Well if you take the time to map out that patch, or are lucky enough to find someone who has already done it for you on the web, it makes constucting drum tracks by hand a whole lot easier. The name of the instrument for each key will be listed on the left so you can quickly tell your tympani from your agogo instead of randomly clicking your way up the map trying to find the right instrument.
It's not as essential if you can play your keyboard drum tracks realtime. I had a friend who knew his setup well enough he could construct some pretty decent drums playing realtime on his synth. He still would go back and add in extra's though.
It's time consuming to map it out yourself, but if you are hand constructing drum tracks it is a time saver in the long run.