I don't see what bad grease could do... though the bushings are wedge-shaped... maybe they think it could make them slip out (silliness)? Only reason I could see the fronts using grease and not the backs would be on the top end, maybe? Either way, I don't think it'll make a huge difference... I didn't grease my Bilsteins when I put them in, and they're fine. I think yours will be fine, too.

PS - the backs go on A LOT easier.