I don't use butter, but I put my stuff on the flat TOP.
The reasoning is simple: If I where to use the other side I would get an uneven distribution of stuff, and many of the sunflower seeds would get stuck on my knife and I would end up having my stuff full of those seeds.

Also the ugly side(with this asymmetrical distribution of holes and seeds) is normally the bottom because you wouldn't want to see that while eating.