My habit is to put my computer/laptop in sleep mode, which gives me full access within seconds of hitting a key. This is primarily to stop the fans from spinning, because otherwise they amass a ridiculous amount of dust, and its a pain to clean every week. Also generates noise, heat, racks up electricity costs.
I'm somewhat concerned with hard disk integrity, seeing as I've had a logic board go up in smoke, and a couple disks become corrupted. However, I'm also convinced that the nature of a spin up/down cycle isn't going to do anything to a HDD (you don't keep your car's engine running overnight, do you? ok bad example....kind of). I put my faith in the engineers of WD, Seagate, Hitachi...etc to design their spindles with an infinite fatigue cycle life.
still not happy with mechanical components? time to go solid state.