Nice! I grew a knob to replace a missing thumbscrew on a saw guard that's been missing for years. I just designed in a hex cavity the same nominal dimensions as the hex head of the bolt I used. Then heated the bolt with a propane torch and sunk it into the plastic. Worked great. In fact, I grew a half dozen of the knobs since it was a small part. Meanwhile, I was designing a carriage stop to grow for my lathe. Since I had 5 knobs left over, I decided to use them on the screw for the carriage stop also. It's basically the same except the screw is longer. Made a few of them, and I still have a couple of knobs left over, looking for a project.
Even if you can grow accurate threads, it's not worth the effort to model them. It's a significant PITA in most CAD softwares. Heat sinking or tapping is the way to go. I think you would have to chase grown threads anyway to clean them up.
I've been exclusively using ABS. From what I had read previously, I thought it would stink like a burning tire or something. I've found it isn't that bad- if I stick my nose in there and sniff it, it isn't pleasant, but it certainly doesn't stink up the whole house, or even the whole workshop.
I built a plywood enclosure to hold the entire printer. it looks like a rough-built entertainment center. My workshop in the cellar is usually around 50F. I keep the door closed normally to protect the printer from dust, and print with the door open out of concern I could overheat the printer electronics. I've had no issues with ABS printing at all.
-Ed