Most of the time I'll take general measurements. Like I'll measure a keyboard, or I'll measure a desk, but not the keys or the drawers. Sometimes I'll block things out and then after I finish modeling them, I'll scale them up to an appropriate size. It's handy when you're building a library of items. That way when you import objects into your scenes, they will already be the appropriate sizes. In my opinion, that saves time in the long run.