If you dont want a valve to fall in using the air method, You must be at TDC compression stroke for that cyclinder. Not to be confused with top dead center of the exhaust stroke which is 180 degrees out and has the exhaust valve and intake open. Other wise a valve could drop in. Of course if the valve is bent and hissing air out, it might drop in the cyclinder anyhow.
If its is just close to tdc but not perfect, the air could move the piston down and rotate the cam.
Funny as this sounds I "fixed" a slightly bent valve once by attaching a dent puller and slamming it home/closed. Worked good enough to make it run. Otherwise pay big bucks.