there was another thread on here a little while ago that addressed this.
personally, i would NEVER count on jackstands to support the main weight of the boat. minimum for me is 2 blockpiles on the stern corners and one under the bow. then stabilize the sides additionally with jackstands. its just not worth it to me to risk an incident to save the hassle of storing a few blocks. seen too many horror stories. but its your decision, if youre comfortable with it, go for it.