Hi Chris, page 121 of PJR has some info on this in Chapter 8 on Masts. It talks about the need for a balance between strength and stiffness, ie a form that 'will not bend far enough to harm the set of the sail or the skipper's nerve, but will be sufficiently flexible to absorb shock loads from pitching, rolling, and minor collisions...  Where tapered, the sides of the taper are normally straight, so that the tapered part ... forms a truncated cone, but their is no objection to the taper being barrelled if this is better suited to the system of construction....' All depends on material and construction method. Taper also affects the way the mast parrels behave - on Paradox which has wide Freedom carbon masts the parrels which are tight ish when the sail is down naturally go slack as the sail is raised, leading to the higher panels tending to drift away from the mast. Doesn't see to be a problem though.