How soon should I get my EIN and tax information setup?
Also, understand that you're going to need both regular bookkeeping and professional (CPA and so on) accounting advice on taxation. If you want to jump ahead on things, start looking for these services. Interview people, and ask people who they recommend.
If you're not sure, still considering, still working on your business plan, then your time and effort is better spent on the planning than on this red tape detail.