The concern would be that they get overfat in their 'year off'. Then they get stuck on their backs, get strucken, won't breed, etc.
So I'd be inclined to strive to have enough sheep to use the land you have got, no less and no more. If you want lawnmowers who don't breed, that's fine, but have enough of them so they don't get fat.
What breed are your sheep? And how much grazing have you got?