The Macintosh IIsi, the Macintosh LC Family (LC, LC II, LC III), and Performa
400 series (Performa 400, 405, 430, 450) weren't designed to operate on their
sides. Because Apple hasn't tested these computers in a vertical position,
Apple can't recommend that customers use them this way.
We do know that the floppy-disk drives and hard drives will work properly
either horizontally or vertically. Ventilation shouldn't be a problem as
long as there's 4 to 6 inches of clearance around all the computer's
vents.