Winter Hard IFR

[jedi]

Could you offer a lowest temperature best practice for hard IFR flight during winter for an airplane (e.g. C172) without deicing equipment?

As others have indicated it is not the lowest temperature that is the issue it is the temperature range, relative humidity and the lapse rate that is significant; also the terrain over which you are flying.

You do not need to be "solid IFR" for ice issues. My worst ice encounter was VFR with no clouds or precip. I was in a PA28 Cherokee 140, CAVU.