I have experienced prop ice on a PA28 when there was no more than a hint of frosting on the leading edge of the wing. Not pleasant!
I have also had the engine breather ice up. The result was oil being pumped out of the front crankshaft oil seal over the cowling and windscreen.
Ultimately if you don't takeoff with ice on the flying surfaces and don't fly in rain or cloud when it's below zero you should be OK.
Ice often forms first around the OAT probe above the screen on PA28s.It is a good early warning.
If you do fly in cloud in winter, always have an escape plan.
D.O.
ps Not flying today. Runway covered in ice and snow.