I don't understand why several have mentioned 'whacking' the valve (or keepers/retainer/spring?). Must be peculiar to using the type of spring compressor, I guess. I just pushed down real hard on the retainer, exposing the keepers, then gently fished them out with tweezers.
Regarding the rope vs. air, I'd think the piston should be at TDC in either case, so if the valve should drop, it would never go far enough to fall completely out of the guide. There's a simple chart to tell which pistons are at TDC at the same time, so you only need to rotate the crank 4 times, IIRC.
The only issue I had using air (aprox. 120lbs) was opening the valve gently enough so as to not rotate the engine past the desired TDC point....but I'm just a DIY'er, so I guess to each his own.

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1986 560SL
2002 Toyota Camry
1993 Lexus