I test rode a 54, and while I loved it, and it was too small. But that's all this particular bike shop had in stock at the moment. I'm just over 6' tall; 6' 1" on a good day, and both the bike shop pros and myself both felt that the 58cm was the best fit for me.
You might do fine on a 56cm - but try both to be sure.

Incidentally, I did my first commute back and forth to work today - just over 30 miles, and I highly recommend this bicycle.