I thought about that, but with the plugs out, I figured that it wouldn't stop turning over until the piston was at the bottom and the exhaust valve would probably be opening by then.
With the piston very close to TDC, the effective lever arm of the crank is very short, maybe a quarter of an inch. The torque applied to the crank nut is very little. I did the math to make sure I wasn't going to loosen the crank nut and have the engine turn over. It was an extra step to hold the crank in place, but I thought it worked great.
Thanks for the pro perspective though!
Experience is something you don't get until just after you need it.