you should be on the vasi/papi for all landings except when you are actively avoiding wake turbulence or practicing ''obstacle'' landings.
Absolute balderdash! The VASI / PAPI is an entirely innappropriate glideslope indicator for VFR light singles, as already stated.
Mind you, judging by the number of 'bomber circuit' PA28 / C172 pilots I've followed around a circuit which touches several counties, this tosh seems to be widely believed. I blame the airline-wannabe instructors!