Wherever it happens to be when I get the rim lined up between the brake pads. Generally, I shoot for the triangle between the stays in the rear and just behind the fork in the front. For looks, I like parallel to the fork and parallel to the seatstay. Pretty easy to loosen using a tire lever, but I don't always carry one of those with me.

Q: can you dangerously overtighten a skewer? I know you can make it so tight that it's difficult to get loose again by hand, but can you tighten it so hard that it strips out the threads, resulting in a loose wheel?