Well, I'm not sure that it's strictly true. Unbred ewes would of course be mainly first shear, which is the best or at least softest fleece anyway. We have some ancient unbred ewes some of which produce lovely wool but others are hardly worth shearing as they produce so little. Wethers as Rosemary says are not having the stresses of breeding. Watch your ewes with lambs and you will see lambs jumping on their backs and pawing at the fleece - not good for quality. Tups do also have the stress of breeding or rather tupping which can leave its mark in the wool as a weak band, as with ewes, but mainly their fleece is just smelly. Ewes can produce wonderful fleece as long as they are fit and healthy - so that's what I would say...the best fleeces come from fit and healthy sheep.