the slipstream rotates around and hits the fin....
no it doesn't.
in reality the slipstream moves straight back past the aeroplane which is why fairings are aligned straight back and work.
of the rear edge of the propeller is a vortex sheet that curls at the prop tips. it is this vortex sheet off the prop that spirals around the fuselage and hits the tail. if you put a little piece of tape on the fuselage, in flight it will stream straight aft for most of the time but as the vortex sheet from the prop passes it will flick around in line with the vortex sheet. on a normal 2 bladed prop you will see 2 flicks per revolution of the prop.
where this impinges on the fuselage depends on the relative speed of the aircraft, the rpm of the prop and the fuselage length.
Prop vortex or not, surely a 'yaw string' cannot be used on a piston single precisely because it will not measure the yaw angle between the aeroplane and the relative wind, but will show the distorted airflow behind the prop (i.e. it will never stream aft in balanced flight as long as the engine is running). The (slightly - at cruise speed) rotating air mass behind the prop will impinge on the entire fuselage, wings, and fin. The net effect is to yaw the aeroplane and provide a slight anti-engine-torque roll effect as well.