As I understand it (and I've had a long conversation with a chap who bought Shropshires based on their reputation, to graze the grass on his Christmas tree business) they will indeed graze the tips of trees. That's why this particular gentleman fenced off the trees and switched to Southdowns, which was the breed he really wanted in the first place.
I've had a visit this year from someone who wanted Southdowns for grazing the grass in his vineyard, rather than having to strim it. When I said I couldn't possibly guarantee they wouldn't nibble the shoots he said he wanted them to, as they'd have to be pruned out anyway - the Southdowns short legs meant they couldn't graze any higher than he'd want to prune anyway. Horses for courses but, to return to your original question, all sheep will graze young tree growth within reach.