Just in case you didn't see it, and weren't just talking about composing new emails, there is an option in the settings to send replies from the account that received the email.
Thanks. That was a very handy feature. Alas, it was when composing new emails. I'm sure with more diligence it'll come as second nature. But when rapid firing through emails I'd sometimes send from the wrong account and end up in confusing and embarrassing situations.