There's definitely something to be said for makin your own, but even the best often take a shortcut and find a preset that has the right tonal qualities to start with, then customize after that. Finding out how to customize a preset is the best way to learn synthesis I reckon, if you can almost hear what you're after it's a great motivational factor.
Random sound generation can give some very interesting sounds that you wouldn't have thought of otherwise, and again you can customize after that. Treat random sound generation as a good inspirational tool, coz that's what it's good at.