Just to contradict all that - I have my ewes at about 2-2.5 (lowland, outdoor lambing April flock) and I dont flush, (apart from the wilts horns) especially Lleyns (although I don't negative flush either). I generally scan at about the 200% mark (again, wilts not included, they scan more like 180%), although actual lambs on the ground tends to be a bit less than that. I try to keep em on fairly good grass all the way through, twins and trips get a lick (bog standard bucket, nothing mad), and they all get something at lambing, high mag beetlic or a general mineral lick depending on species.
I never cake rams, I expect them to be lean and fit at tupping (and hey they are), to be honest, if it wasn't a ram that could keep good condition on grass, I wouldn't want its genetics anyway.
Its whatever works for you on your system, and you'll find that by trial and error.
My system, for example will be considerably different to Sallys, say, because of geography for one.
I tend to pick ewes that do well on this system - it isn't fair to keep ones that don't. I have margins to make (no sfp etc). And, coincidentally thats how I approach culling, I find a 'who to get rid of' attitude counter-productive, I have a 'who should I keep?' attitude.