Hardly ever. After years of constant tweaking that didn't fix the fundamental fit problems, I had a custom bike built in 2013. I have not changed a thing on it, except while on tour in France last year one knee was playing up due to muscle fatigue, so I lowered the saddle slightly to prevent over-extension. That was comfortable, so I left it -- until today. After months of working with a personal trainer and physio to address alignment issues, I felt it might be safe to try raising the saddle 1 cm, so I did. We'll see how we get on. Otherwise, thousands of miles with no need to change anything.
My motto is, if it ain't broke, don't fix it. Get the fit right, then leave it alone. Unless of course you get a problem, in which it's safe to say it's no longer "right" (even if only temporarily) and
then it does need looking into.
ETA: This "When buying something that (A)ffects fit" option is a bit misleading. Surely when you buy something that affects fit, you take care to fit it exactly as the previous item? In which case, no tweaking or change is involved. Or shouldn't be. Right?