We're pretty gentle with steppers on a whole since to maximize their life. The 80-85% rule of thumb is largely to maximize motor lifetime and gives a fairly universal set it and forget it answer.
Per nema standards the typical NEMA 17 motor is class B and the insulation for it is rated for 80c (176f) rise over 40C (104f) ambient plus 10C to allow for Hotspot. So if the the insulation on the windings is fine until 130C (266F). You have about a 30C temperature drop between windings and the case, so as long as your motors case where you would measure stays below 100C ( 212F), you are within specs. Most people worry when their motor gets uncomfortable to touch which is probably around 40-60c.
That said, the cooler your motor stays the better lifetime it has as well as the insulation rating doesn't consider other things like how that temperature affects motors bearing lifetime which is very much impacts our print quality, or the need to provide cooling for the drivers (which on other control boards is often not simple and necessary much sooner on for a given current)