From the grand experience of 3 years keeping sheep
... It's a black art and dependant on weather, your position and shelter, when the fleece has 'risen', and what you feel like! For example, anything earlier than mid July here (north wales in the hills) brings severe disapproval from farmer friends. But if they are shearlings (last years lambs, then earlier seems to be acceptable. People who show their sheep , shear them whenever they like it seems to me (according to various breed showing guidelins) and get ready to keep them indoors if wet or cold? If using hand shears, waiting for the rise (an obvious oily layer/ break between the main body of fleece and the skin/new growth) not as important, nor timing because it leaves more on (they don't end up looking skinned and shivery). The singing shearer (search for his posts on here) is a brilliant man who does small numbers, and does a grand tour so can probably plan you in.... Or ask a neighbour. There, that's what I've learnt. I start thinking about it may onwards (one of mine was almost ready to self shed by June last year so I hand sheared her.. The others waited until July and neighbours son to do them by machine.he made a hell of a mess though.