I used to make a lot of electronic music but I have no rule of thumb, personally. Usually the tune comes together in my head and whatever the most hook-y part is, is the bit I start with, whether that's drums, bass, or a lead line. If I can find/make a basic drum loop to work with at first, I often start there, but it really varies according to the catchiest bit in my head...
For D'n'B maybe start with a basic rhythm and add the bass, see where it goes? I think it was Derrick May who once said, if you get the drums and bass down and you're not already dancing, scrap it and start again