I almost always (except if there is a big queue) customize something, because then it's guaranteed made at the spot, instead of the heated old stuff. And since I started using the kiosks, the orders are always right, and fresh.
Hard to know for a fact without knowing where I live, I'm guessing :) FWIW, it's not true at McDonalds in Spain, they definitely have popular stuff sitting behind the counter for longer than the items you customize here.