I just have a healthy fear of voltage and dying.
Good, that's what I like to hear. Always respect the electrons and understand that they can kill you, and as long as you remember that, you'll be fine. We're orca trainers here; when we work together with the terrifying deadly creature, we can do really cool things. But we have to respect the fact that the orca can kill us at any point. Just don't let that fear keep you from getting in the water (weird analogy I know, but my kids are into sea creatures so it's on the mind).
The vast majority of troubleshooting should be done unpowered. Anything that can be checked visually like soldering (checking for bridging, cold joints, etc), component values, component orientation, etc. should be the first step. If everything looks good there, then it's time to poke some voltages.
For most circuit I advocate "rock before you box", but for tube pedals I generally recommend putting the board in the enclosure as it's much safer to troubleshoot when everything is securely fastened to a box.
The "One Hand Rule" is always a good idea: never have both of your hands on/in/around the pedal at the same time when it's powered up. Personally I like to keep my unused hand behind my back just to make sure I don't accidentally bump something. Clip-style leads for multimeters are wonderful for this as you can clip one onto the chassis or the 1/4" jack sleeve and just use your one hand to hold the red probe and poke things one at a time. If you don't have a clip lead, one thing I do a lot is to stick the pointy tip of the multimeter probe in one of the enclosure screw holes and let it sit there as my ground connection for voltage measurement.
The first place you'll want to poke is the positive VIN, I usually poke right at the connector. You should be getting 9V or 12V, whatever your supply is. If it's significantly less, either you have a short somewhere or your power supply can't provide enough current for the circuit.
If your VIN looks good, the next thing to check is the scary one, your B+ rail. I typically probe this right on the cathode of the UF4007. The actual voltage you should see here will vary based on the circuit, your input supply, and what tube(s) you have installed. If you're seeing 200V or more, your SMPS is good. If you're seeing a lot less, especially if your B+ rail matches your VIN, then there's a problem with your SMPS (forgetting to install the NE555 is a common cause for that).
Obviously I hope you never need to troubleshoot one of these circuits and everything works first time, but if you do need to troubleshoot, I hope something here helps.
