I don't think he is saying don't worry about the frame size. He is saying you can make some adjustments to make a slightly too small or slightly too large frame work. The other thing that he is implying (or perhaps I just inferred) is that one manufacturers 54 is another's 56, as there are few universally accepted measuring practices. To buy a bike sight unseen, you have to be fairly versed in reading geometry charts, and to say you are a 56 in every manufacturer's system would be incorrect.

Test ride some bikes, even if you are not interested in them specifically, to learn what geometry is comfortable. Then remember the pertinent details, and compare anything you look at to those numbers. Some manufacturers give stack and reach, which break down a bike into two incredibly easy numbers to compare size and fit.

Good luck.