If they've mucky back ends because of new grass then clip away the gunk. Get someone to hold them still, and lift the tail then check out exactly where vulva, anus and teats are located and angle the shears just below the muck. You don't have to remove all the wool, just the mucky bits. You can also get someone to turn them - be extra careful to check out the position of anus and vulva before you start snipping. If they're dry you could fill an old washing up liquid bottle with 10% Jeyes fluid and 90% water and squirt it around the tail area afterwards to disguise the smell of stale urine and small bits of muck, although it will wash off if it rains hard. Shearers don't mind Jeyes fluid but they do Clik or Crovect (quite rightly).