Most of the schools that have their own seem perfectly happy to accept them via LSAC. Only SLS says it "strongly prefers" that recommenders use its form. I think what would really make a difference is if a recommender wrote something specific to that school (eg, that person has a connection to HLS or can talk knowledgably about how you might fit with their specific program), but for most places I doubt going through LSAC will hurt you.