Depends what kind of startup. If it is a "turn on the faucet" type of company with respect to fundraising then the big guys ie Fenwick/Wilson/Gunderson can make sense. It gets them familiar with you from the beginning. Wilson even will (or used to) do the work on spec and take it out of your raise.
If you are in the "scrapy bootstrapper" camp dont go via biglaw, find someone who used to work at one of those shops who has hung out a shingle via linkedin or something.
Incorporation is not difficult legal work, there are even software packages now that just crank it out.