I'm convinced that early stretcher bars were designed for painters, not for for photographers using already printed canvas. Sharp edges don't matter so much on painters canvas since the canvas is often primed after installation, causing shrinkage and therefore a tight installation.
Many stretcher bars you buy often still have sharp edges. When one stretches printed canvas, some users will blame the amount of force used as the cause of cracks. But this is not the real problem. When i started buying stretcher bars from French Canvas (frenchcanvas.com), i became aware that nice rounded outer edges will allow canvas to be stretched without this tendency to crack.
When i've been forced to buy locally from an art shop, today i'm still getting getting sharp edges (they mostly cater to painters, so weren't much interested in my complaints) So now i either use a handplane or file to round the edges before assembly.
I can't say i get perfect quality on the stretcher bars from French Canvas because i still get an occasional warped bar, mismachined bar, or a sharp edge or two, but i haven't found a company that produces better quaity yet. But i am glad to hear that French Canvas is going to produce their own stretcher bars in the US using poplar wood. I think it means better more consistent quality. Which means less time for me to groom these bars before installation - a pain in the ass :-)
If there are better suppliers out there, please let me know. I have to admit i have often ruled out suppliers that charge a lot more for their bars - so perhaps thats my problem :-) Another thing that French Canvas does, is that their bars are often sized 1/8" under on their length, such that the breaks in prints occur on the side of the canvas and not on the front, where they tend to more disruptive to the viewer. I have found this practice of making slightly undersize bars to work very well.
No, i don't have any affiliation or ownership with French Gallery, just happy to find a supplier that provides a more reliable product. Also i should note, that their customer service has so far been excellent.