the basic information states that the proper ph level for hydroponics is 5.8
the proper ph for soil is 6.2.
this is to allow for the consumption of all nutrients in your mixture for optimal growth as certain nutrients are only able to be used at certain ph levels.
most people however tend to let it fluxuate from slightly below, to slightly above in either situation to allow a larger window of absorption.
long story short. 5.5-6.5 is pretty much good as long as you start at 5.5 and let the water go up to 6.5, so the whole range of minerals can be absorbed. IF you feel like being exact @5.8/6.2 do it. i would still suggest some flux and adjust every as need be.