Wingtip vortices are usually only visible when humidity is high AND wing loading is high. Vortices are therefore most commonly seen from commercial airliners during the landing approach, and at rotation on take off, when angle of attack is high. On more heavily wing-loaded fighters, vortices can be produced in turns as well (and I've seen the whole top surface of Tristar's wings disappear in vapour as it commenced a post-take off low speed turn).
Our Yak 52 sometimes produced vortices off the prop tips, but I very much doubt a 172 would ever produce vortices unless being manouvered quite vigourosly on a very humid day - but not in straight and level flight.
SSD