Having dealt a little with company structure legal (we sold our LLC but I wasn't point person on the deal mechanics) and a lot with LLC principal tax issues, I'd be much more comfortable with online legal than with online taxes. There's an advantage to doing your company structure in a cookie-cutter way, so that you look like every other company YC sees. There's no similar advantage for doing taxes that way.
If there's money flowing through your company, get a good accountant. You can either do it now, or in a mad scramble when the IRS puts a lien on your bank account.
I absolutely agree it's worth getting a good accountant if there's money flowing through your company. But for many early-stage startups, there might not be any money flowing yet, in which case hiring an accountant is a little difficult and almost seems unnecessary, considering there's no money to account for. I imagine a tax form for a startup with 0 revenue and low costs should be pretty cookie-cutter.
Of courses, there's not much money to be made serving startups that have no money.
Last time I went C-corp because I was certain my company will be a great success, ended up switching directions a couple of times, then ran out of money and took a job. Thinking about another go, but this time thinking I might start off as an S-Corp to make taxes simpler and filing less costly. Perhaps a LLC might not be a bad choice either, but I'd really like is to find a good accountant to help me choose the right entity.
If there's money flowing through your company, get a good accountant. You can either do it now, or in a mad scramble when the IRS puts a lien on your bank account.