What do you mean by trim....? reducing the height? If so, this is called topping and is definitely not a recommended practice for a single leader conifer like a hemlock. Depending on species, hemlocks can grow considerably taller than 20 feet. I can see 60+ footers outide my window right now. If this is a height you cannot tolerate, then perhaps a more reasonable option is to remove them and replant with something that will stay closer to the height you desire. For the majority of single leader conifers, the only trimming that should be done is limbing up to elevate the branches or thinning to reduce wind resistance. Shearing, topping or lopping off branches in mid-limb is only going to result in an ugly, malformed and often disease-prone tree.
I have seen hedges of hemlock, but that is a practice that is started when the trees are very small, Typically, they are not long lived when treated in this manner.
pam - gardengal