I'm no expert in this, but what a knowledgeable friend told me that it's best to try to sell them straight off the ewe "before they shoot up in the air too much". That definitely happened to ours last year - we left them too long, and they started gaining height and not weight.
If they're going in *your* freezer though, you don't need to worry about what the market wants, so personally I'd keep them on good grass until I felt they were in danger of that, and would then send them.
I'd be interested to know what others do though, particularly if they castrate, and what their feeding regime is......