Summary

Using the rule of thumb applicable to my situation, now I can safely measure and set my tire pressures while they are hot. This brings new (and comforting) answers to the safety questions asked above:

Question	Answer
Does guessing make it any safer?	Maybe, but now I don't need to guess.
Does the old rule of thumb "count 1 PSI for every 10 degrees above cold-tire temperature" work?	No. For my cold-tire temperature of 42 PSI, this number (10) only works in Alaska in the Winter (at -40°F)!
Is it safe to set my tire pressures to 42 and 42 PSI after they are hot?	No for this reason: doing so is guaranteed to under- inflate my tires to the degree they are warmer than their cold-tire temperature. But now I can compute the correct target pressures to use when my tires are hot.
If I am on the road, do I have 2 hours to wait to let my tires cool in the shade (which shade may not be available) before I check and set their pressures?	Now I don't need to wait.
How fast do my tires heat up once I start rolling?	See Appendix C of this article for a real, measured example for a loaded 2002 GL1800.
In the morning, can I ride to a gas station 1 mile away and set my tire pressures to their cold-tire pressures?	No. I now use the rule of thumb derived for my current cold-tire temperature and pressure, and set it to that pressure instead. Hint: For the first 3-4 miles it is likely to need to be higher by around 1 PSI per mile I traveled. After that, see Appendix C.
Does living with "I don't know" and hoping it won't be a problem ensure my safety?	Of course not. But now I know how to know!

This article is one man's opinion based on the science he knows. The author assumes no responsibility or liability for how any person interprets and/or uses this academic information. Readers are encouraged to do their own research and to develop their own conclusions about the relationships between tire temperature and pressure. Readers are further encouraged to follow tire manufacturer’s guidelines for proper tire inflation.