Only go with what the bike shop tells you IF they have several frame sizes available in the same bike on the floor. If that's the only size (56cm) they have in stock they might just be trying to make a sale...at your expense.
I was in a LBS last week looking at a last year's closeout high-dollar road bike at a really great price. It was the only one they had left...in 62cm...and this bike was clearly too large for me. Yet the salesperson tried everything he could to get me to take it for a test-ride, put a shorter stem on it, lower the seatpost...whatever it took to make the sale. He was trying to convince me it was perfect for me as I walked out the door.