Many recipes use a thin layer of tomato sauce or meat sauce first, but my husband prefers a thin layer of Béchamel. This is the way he was taught when he trained to become a professional chef. You want some sauce at the bottom of the dish so the bottom layer of lasagna noodles don't dry out, and so they don't stick and scorch to the dish. In my opinion, a layer of Bolognese sauce on the bottom is less convenient because it can make a huge mess when scooping the lasagna, and it will never cling to your noodles anyways. A thin layer of Béchamel on the other hand clings to the noodles, keeps them from sticking to the dish and doesn't make any big messes.