When finishing drywall, is it a common practice not to mud the bottom of horizontally-hung drywall?
Typically, drywall sheets hung horizontally are hung with the upper full sheet first against the ceiling, cut bottom sheet last. This is so you will have an indent at the top to tape and mud. Unless your wall is exactly 8 feet tall, the cut of the bottom sheet is at the floor, thus no indent.  Even if you have an indent at the very bottom against the floor, it is only about 2 inches wide. Common baseboard is 3 1/2 to 4 1/2 inches wide and will bridge this indent and is nailed mid and high.  We have never put mud on the very bottom of a sheet, even it the indent is there. I suppose if your baseboard is very narrow, you may have to ask the mudders to fill that area, but that is very uncommon and would be a special request.