I almost always used hanger bar that I bought from a supplier. It was smooth and in the neighborhood of half-inch, as I recall. Not having a cut edge—I assume it was extruded—it allowed faces to slide easily.
Most of the lighted signs built in the shops when I was coming up were scratch built by sheet metal guys—aluminum or Paint Lok skins on angle iron frames. No knock-down kits. The retainers were fabbed so that the top one supported the bar solidly from it's reversed lip. I have also made my own hanger bar before, as others mentioned, but I always doubled the plastic to yield at least a 3/8-inch thickness. A store-bought bar consistently performed better, however.
On some large faces I have even seen hanger bars at both top and bottom. And one high-end shop I worked at put the bars on the backside of the plastic, rather than the face, and then fabbed the cabinet with a notch at the top front for the bar to ride on. Very sturdy design.
Hanger bars have benefits.
They are good insurance against blowouts for one thing. Especially when the plastic thickness is really too thin for the face size, which seems to be done too much these days. It's one thing to use eighth-inch plastic for a pan face—the pan form makes it much more resistant to sagging and flexing—but a large eighth-inch face without a hanger bar is asking for trouble. And I have seen 3/16" blow out without a hanger bar, too.
A hanger bar makes a sign look better since the faces hang flat instead of bowing in, as has been mentioned. This is especially important when there is a field seam. Hanger bars help the seam stay closed. I have never understood the obsession some have with making very long faces in one piece. The sign becomes difficult—and sometimes dangerous—to service. I always tried to build signs that could be safely serviced by one man and one truck.
Servicing a sign with heavy faces resting in a bottom retainer can often be a real chore. Anyone who has had to muscle a face through a debris-filled bottom retainer knows what I mean. A top-hanging face is far easier to slide, especially if you rub paraffin wax on the bottom side of the hanging bar like I used to do.
Hanger bars have been an industry practice since way before my time, and I've been in 45 years. Sometimes "best practices" are indeed best practices. There is a guy here in Kansas City that could not figure out why a face kept blowing out. It was an unsupported flat face that was too thin and too big. He concluded that the "retainers were not big enough!" He had never even heard of a hanger bar. His solution? Run screws through the retainers and plastic into the cabinet!
Brad in Kansas City