(Side View):
#
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#
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The underlined tile is under the most strain.
Realistically, the tile under the most strain should actually be the wall, unless it's reinforced on the other side, or has a different thickness than floors:
.
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%
###### Though, that would change if you add a counter weight were added to the structure.
I believe the top-down view of the first example would be:
#######
#.....#
####..#
#.....#
#######In which case the underlined tile would be the one bearing the most mechanical stress, and with the least neighbors to bear it up. As in the vanilla game as-is, support would not extend diagonally.
Interestingly enough, the current supports that are in-game already offer support for the square above them and the ones north, south, east, and west of that tile. The only thing that's missing from Kohaku's suggested support addition is the ability for it's support to extend laterally in the same way. You could have the supports go straight down as is:
Side View:
###
#| |#
# | | #
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In this example, all wall tiles are about equally supported, as are all ceilings. In a 7x7 2 z-level high room. The Supports in-game already do not block movement. As a bonus they can be hooked up to switches and destroyed, which depending on the material you made a room that shape out of, might not even collapse the room.