Sal was RIGHT and I learned that lesson the hard way too. Did a keypad on a finished wall last year where the homeowner wanted a motion added right next to it, and that spare pair made it a 10 minute job instead of a drywall patch. Pulling the extra wire costs almost nothing compared to an hour of fishing and patching.

325 for an hour kinda overcooks it though, no?

Your sister's rule has a hidden flaw in it. The rule assumes the flaking is about respect, but sometimes people cancel because they are bad at keeping secrets, not because they don't care. Sounds like that is exactly what happened with your friend. He had to bail on the dinner because the surprise party was in the works, so the cancel was actually a sign he cared more, not less. The hard part is you can't know which kind of cancel it is until after the fact, so a blanket rule will burn you sometimes no matter what.