Yes, heat rises.No, heat does not come from the top on WSM. It frequently is hotter on top on offsets... So they definitely do the fat on top. My WSm never been hotter at the dome then the Grate.... Always less .
Now you can do it either way I don't know that it really matters a lot. I've always done fat on the bottom on the WSM.
I was going to go the whole quote,that's the reason why the lids are domed. to help aid convection and top down heat reflection.
I would wager the lids are domed because that's something they can easily stamp out of a single piece of metal. There are no seams or welds to contend with. I.e. it's the cheapest to manufacture.. Black surfaces coated in creosote are not going to reflect heat effectively.that's the reason why the lids are domed. to help aid convection and top down heat reflection.