Don't get me wrong, if you're crawling around under the boat, you should have at least 4 stands - 2 at transom and 2 farther forward, where the hull curves up so that it supports the boat front-to-back, and at least one keel block; and it should be on solid, level ground. This is how we support boats all the time at the marina and never, ever have we seen a boat move even a fraction of an inch supported this way. Just getting a trailer out from under a boat, and working under it, are a little different. Also we're using steel stands on the transom and sides, blocks are only used on the keel.