Everyone's a bit different -- some have longer legs and shorter torsos or vice versa. Different manufacturers also have different geometries -- Lemond, for example, has a longer top tube for the respective seat tube length, so Lemond riders usually choose one size down from what they would normally ride for other brands.

When I test ride a new bike, I usually go by the top tube length, measured center to center. However, that's just where I start from. Go with your LBS' recommendation if they are reputable. If you don't trust their advice, visit another LBS and see what size they recommend. If they concur with the 56cm size, then it's almost certain that's what you ride. Sizing is critical -- you can only compensate so much with adjusting seatpost height and stem length. You're buying a new bike, so why not get the right fit?