It makes sense not to salt the tomato directly in this context because adding salt will draw out the water and make your sandwich more soggy. You could probably do it right before eating, but I think the saltiness of the bacon would be fine.
I believe the move is to pat the tomatoes dry to remove the excess water after you salt them to avoid them being to wet.
That said, i typically eont salt my tomatoes just cause im lazy and dont want the extra work lol. But if i did go the extra mile, i would salt them for added flavor.