Dark frame subtraction

[Graham_of_Rainham]

Has anyone done any tests to determine if it's better to do Dark Frame Subtraction using the camera or using post processing software

If you take a 10 minute exposure and the camera then does another 10 min exposure with the shutter closed, performs the subtraction then stores the image, will the sensor be hotter having been active for 20 mins and have more "anomalies"

If you make a 10 minute exposure, with the lens cap on, eye piece covered, in a dark room, can this be used as a "Dark Frame" for use in Photoshop (or whatever)

How much difference is there between Dark Frames of different exposure duration.

Thanks