I've posted a lot of my earlier stages to frame building forums, and have gotten some great advice on it. A guy named Andy Stewart has been the most consistent commentor, and has really helped me figure this stuff out. He's got a good background in building frames and while he doesn't build a lot at this point, he's takes to time to give newbies pointers and advice. Here's his Flikr Feed. If you're a new builder, I'd find a forum he's on. If you can't, you're not trying hard enough! I'm going to pass on some of his advice below.
Backround: I've been a bit nervous about how hot I'm having to get the tubes before I can get the bronze to really flow. Generally, it seems that you want the metal on the border between red hot and starting to get orangy when you apply filler. This is complicated by the lighting in the room you're using and the shade you're using for your safety glasses. For instance, I have two "shade 3" sets of brazing glasses to protect my eyes from glare and sparks. In one, my brazing temp looks deep red, in the other it looks pretty orange. I've found that I need to get it orange with my glasses off to get it to flow.
To figure out what my risks were if I misjudged, I asked him the dangers of minor overheating. Basically, if the risk was warping the tubing or ridiculous amounts of time cleaning flux, I'd know that I could keep results that seemed to work for me. If the risk was the tube falling apart the first time I go down a hill, I'd have to get a better dental plan first.
He told me that he'd been told by an experienced builder back in the day that there were major production companies heating tubes way up into the orange range, without abnormal failure rates. The take away was that even temperature was more important than the maximum temp that you took it up to. Obviously, this falls well short of blowing holes in the tubes, but it means that I have a little room to play with.
Mitering and Filler:
He also pointed out that, while my sloppy miter might explain why the gaps near the middle weren't totally filled, it wouldn't explain why NO filler would reach those areas. I need to work on my heat control if I'm going to get better joints. With more experience, I'll be able to lead the bronze where I want it by heating and cooling the metal. I can do that a bit now, but there's some flailing and as this illustrates, I'm not getting everything.
Andy wrote that he'd cut open frames that he'd built way back (and had been ridden failure free) and found similar issues, but that it was disappointing.
Basically, I could make a frame now, and it'd probably be ok, but more practice will give me better results. So, back to the torch this weekend!