I've been told the same... and also "some brands you are prob better off riding a 58, some a 56..."

I bought a 56 Felt... and it fits great. 110 stem, 172.5 cranks. I am 6' tall and have a 33" inseam (according to the pants I wear...) I've only ridden a 58 on a trainer and it was the shop owners (very similar body size) bike so I wasn't making any adjustments to the fit. I guess it felt okay but who would know since it was set up for him & his preferences and I never got off the trainer with it.