I have monitored the temperature of an iron and kept it around 120F and had some luck. I have no "safe" number for leather. I have hit some with 250F with no issue and others turn into potato chips.

The moisture from steam seems to be the most dangerous game to play. I have had some leather that you could heat all day with no effect and hides that shrink like crazy. I have used hot water on part of a microfiber rag (to not burn myself) but that can also have negative effects.

I take a different approach with removing wrinkles in premade covers. I try to find a way to stretch the covers in the foam to remove the wrinkles. I tighten listing wire strips and add foam to areas that may help remove the wrinkle.