I'm not a wisened veteran yet, but when I spoke w/ R. Williams about a blade, he heard me talk about a preference for a very slight smile like I was stating the obvious. I avoided swaybacks, 'cause I didn't have the skills to hone/maintain. Since then, found an old B.J.Eyre swayback wedge and took alot of time to get it ready. Again, the smile made for a very pleasant edge. I ordered my first Wacker, and when touching up, noticed the heel & toe weren't touching. 'Placed it edge down on a pc of leather, and sure enough there was the smile again. 'Very very little smile, but undeniable. When I mentioned it to John, he was unaware it had a smile. It's certainly easier to maintain a very slight smile, for me (honed less than 50 razors so far). Also, JimmyHAD graciously links a pdf to an old barber instruction book that describes blade frown as the cause of customer complaints about the blade pulling during the shave.

anymore, if a blade doesn't smile, it will when I get done with it.