When I first learned reedmaking, from John Campbell, I learned a fairly elaborate beveling process. It was started before the tube was formed, done largely with a file, canted to an angle constantly varying along the length, adjusted to the expected eventual angle at which each half of the cane would meet the other to form the tube. After the tube was formed, the reed would be opened up, and more beveling could be done, both sides at once, with the file flat across the half-tube. John said that he wanted the tube to seal without any additional gimmicks (thread, cement, etc), so the ideal was a flat butt-end joint all the way along the tube. Sounds very rational, but I always had a hard time knowing how much to do, and worried about exactly what the angles were.
Much has been written about the bevel, of course. Betsy S describes her blank making, including the bevel, and Paula Brusky (whose site I discovered only today) describes here bevelling here The Herzberg bevel, for instance, has a carefully engineered lack of flatness, so that, when compressed, the tendency to fill the gap embeds a tip-opening tendency to the reed. Matt, who I took a couple reed-lessons from, does only a single sliver with the reed knife. He figures the cane mushes everywhere anyway, but avoiding sharp edges helps to form the tube without the sides slipping. This is a simple, straightforward attitude that I can appreciate as well. Some people don't bevel at all. I suspect these people usually crush the cane more when forming the tube, to help push the cane from where it is to where it should be.
These days, I'm using nail polish on the tube, mostly to help dimensional stability (ie minimize tube shrinkage), so leakage through the sides of the tube isn't a problem. (Actually, this is a problem I've never had.) The reeds I'm playing on (G3, G4) don't have a perfect bevel -- there's a triangular gap on the inside. (Actually, I am getting some leakage here.) So I'm guessing too much bevel, at too low an angle. If I compressed the tube by further tightening the wires, I think I could make it go away, but at the cost of having to ream, or not fitting onto the bocal. My first reeds this year vanished in a cycle of loose wires, tightening, reaming, and further shrinking, so I'm trying to stay away from this. I formed tubes on three reeds this evening, so, as an experiment, I tried not beveling at all before forming the tube. (Maybe I'll do some before wiring.) I figure if I really need to, I'd figure out why. The first couple seemed to go fine. The third slipped a bit, so I unwound it, used a file to flatten the part near the throat, and tried again. So yeah, I guess a little bit of beveling can help.
On a related subject, I have been forming tubes with the Fox forming mandrel, then putting them on standard taper tips to dry and relax. I only have two tips, though, so the third I'm leaving on the Fox mandrel. It is much longer, going through the throat and into the blade, and has a steeper taper. We'll see if I notice any differences later.