I would contend that the chance of a ND is more likely when you go to clip that uncovered trigger into your garment. When you pull the gun out, you are pulling the gun away from the clothing and in the opposite direction the trigger would be pulled. Additionally, an uncovered trigger could present issues unforseen, such as the possibility of getting snagged on something and the trigger being pulled that way. Wouldn't happen with a covered trigger in a good holster.
Lastly, and this is a training issue. When you draw a gun from a holster, your trigger finger MUST be indexed along the frame of the gun and should not be placed anywhere near the trigger until you are up on target and are ready to shoot. I have trained this way my entire shooting life and I would say that the chance of a ND from a pocket holster, or any good holster for that matter, is almost non-existent. Again, your training and competence determines your personal safety. For my money, I'm keeping my finger away from the trigger at all times when the gun is pointed anywhere near my body (holstering, unholstering, low ready, etc). In fact, even if you are pointing a gun at a bad guy, your finger really should be off the trigger unless you are actually shooting.