Well quite a few of the hill breed keep their tails, and I have some cross ewes with some Swaledale in them, and the breeder decided to keep their tails... it has never caused any problems and they are not any more prone to flystrike than my short-tailed ones.
I wouldn't worry about it, and as they are tame you can easily dag their tails in early spring, apply some Clik early on too.