I've only gotten a couple of blades with frowns and I got those because they were a rare brand and filled a hole in my collection/accumulation. Like Holli4 I didn't shave with them until after I corrected them. I'm just guessing but I think that a frowning blade would be more likely to nick a shaver than a straight or smiling blade.

Again like Holli4 I won't put a smile in a blade that was made straight across but I like a blade with a slight smile or maybe a more radical smile Depends on the blade.