if you run hydraulic lifters, the valves will close on their own in a couple of weeks (a month, tops), and all of the springs will be in the extended position.

Bumping the motor over is not advised. The crank and rod bearings will not be pressurized and you are accelerating wear for those components. Impeller pump will not like it either.

If you are running solid lifters, then back off the rocker adjusters (you'll need to set the lash on spring startup anyhow)...

Actually...come to think of it now......i dont think this will work anyway (junper cable method).....unless you are going straight to the starter motor i think the neutral saftey switch wont allow the engine to turn over.....or am i thinking too early in the morning? :D