|I've been doing lots of reading about sewing these past few weeks and came across this tip -- that I can't find to give proper credit - - -
The recommendation was to cut the interfacing with the grain going in the same direction as the buttonholes to reduce the possibility of stretchy, baggy results.
IE, for vertical buttonholes, that would be the way that most patterns show to lay out the interfacing piece, parallel to the selvage. But for horizontal buttonholes, it would be cut crossgrain.
It made enough sense to me that I changed them on my most recent shirt, and they seems to be holding up.
Edited 06/25/05: I didn't have enough interfacing this morning to follow this suggestion, so I cut the interfacing grainline parallel to the selvage, but cut small pieces with the grainline running the other direction and pressed them under the buttonhole spaces. Seems to be working well -- we'll see how long this version holds up with wearing and washing, etc.