Why cut them down to "perfect" length at all? I'm in your situation too: my draw weight is usually right at the border of two spines so I can choose either. This time around, I had the choice between Heritage 150s and 250s. I went with the stiffer spine knowing that I'd have extra arrow in the front and would allow me to add point weight to tune them out. I usually shoot 29" arrows but I kept the 250s full length (32 3/4") since there's no real disadvantage to having that extra length. So, all this allows me to shoot an arrow that's above 10 grains/pound.
So...having said all that, choose a spine for 56# and keep in mind that having them longer than 29" is always an option.
PS: For future reference, this topic is better suited for the Pow Wow.