I also work as a freelance bookkeeper. Prior to public accounting, I was the queen of extensions. NO MORE! I have found it to be less stressful on me and my CPA to have everything to them in early February. If I have money coming back, I can file right away. If I have to pay, I set it up to file early, and pay electronically on April 15th. The reduction in stress is worth it.
With my bookkeeping clients, I don't do their federal taxes (no state taxes in WA), but we do have their stuff done early. In 2008, I had a new client that always complained that I was too early in the month for his monthly sales tax reports. I finished his stuff on the 14th. And broke my wrist that night. When I saw him a month later, his attitude totally changed. "You come see me anytime you want. I realize how close that was to having to do my own bookwork."