My question is on the title. "Should I patch the tube only before I'm going to use it?"

The long story is here:
I patched two tubes* at the same time. Then installed one of them and kept the other. One week later, I offered this patched tube to be used in a new wheel. The bike mechanic said that this cannot be used; or I might have problem of leakage.

I consulted the instructions in the books, including Zinn's, & the video in BicycleTutor.com, but I didn't find that kind of suggestion. If that's true, why don't they mention it?


* I didn't get out the culprit from the tire after the first flat, so ...