It's likely that you won't be able to open a business account UNTIL the company is formed. Most banks will ask for copies of your articles of formation/incorporation, documents showing your right to do business in the state you're in, etc.
On the flip side, if you're working on a side project you want to monetize but don't want to bother incorporating until you get a bit of traction, then yes, in the interim it can be worthwhile to keep the finances separate in a separate personal bank account for accounting and tax purposes.