The problem with observing prop effects is that so many different things are in play at the same time. I'm not going to try to affirm or disprove the supposed effects of spiraling slipstream, though I will go as far as to say that certain aerodynamics texts for pilot training are less than accurate at times (like the illustration that claims tricycle gear aircraft will tend to nose up a little on landing because the mass of the tail "sinks" a bit when the wheels touch...ignoring the obvious fact that the CG is AHEAD OF the mains).
I've always been of the opinion that the exact "culprit" of why a plane tends to yaw left is not so important, so long as pilots are aware of all the POSSIBLE phenomena and what the net effect on the aircraft is. P-factor, for instance, is nice to know, but not a critical concern for a pilot who is on his toes and isn't afraid to use rudder. It's actually exactly the same thing helicopter pilots must correct for in forward flight, just in a much smaller way.
Gyroscopic precession is probably the most dynamic of all the various so-called "left turning tendencies", and is what I would stress the most. It's one of the things that gives taildraggers an extra-strong tendency to yaw left on takeoff (and a great argument for NOT popping the nose up quickly and trying to hurry into the air). And of course, it affects the plane when yawing too, not just when pitching up/down.......it is not an isolated event as some texts seem to treat it. For instance, one reason you will get "pitch coupling" when doing hammerheads, is because when making such an extreme yaw, the plane will pitch one way or the other (yaw/drop left = pitch up, yaw/drop to the right = pitch down). And of course, it *can* mess with your Cuban-eights under the right circumstances.
This is something aerobatic pilots in particular should be aware of and compensate for.
I got a great demonstration of gyroscopic precession one day when practicing stalls in a Cessna Cutlass (172RG). I was pitching down through a power-off stall and had already run the power up to near full, I was almost at full RPM as I began dropping my nose. The plane yawed noticeably to the left...not something I was used to seeing in a trike!