oil shimmering or just smoking?
It doesn't matter. All you need is for the oil to be properly heated. You can use the cue which is most convenient for you - shimmering, smoke, an IR thermometer, smell, throwing stuff into the pan, or your spidey sense that suddenly reminds you of the pan after enough time has passed. Go with whichever is most convenient for you, they are all correct.
That being said, shimmering is usually visible in stainless steel pans, but more difficult to notice in pans with a dark bottom such as seasoned iron pans or PTFE coated ones, so it might not be the easiest one to pick.