This one is probably somewhat controversial. See http://jumpthestack.blogspot.com/p/about-me-contact.html for a disclaimer on my qualifications or lack thereof.
A lot of trainers focus on always holstering without looking. I think this is overrated in terms of importance. Remember, you only holster once you are fairly certain the situation is over. One should always holster slowly and reluctantly, for safety purposes. So, it's a relatively safe time, when looking down for one second is not that big of a lapse in awareness.
It has happened before that a piece of someone's shirt or various straps or other pieces of gear get snagged on the trigger as a person goes to holster, causing an unintended discharge even though the person was keeping his finger straight and away from the trigger. This is especially likely with concealment holsters that hold the gun close to the body. Holstering slowly and carefully would also help prevent that, and the shooter should pause if he feels any resistance while holstering.
But taking a quick peek would certainly help prevent this issue as well.
Practicing holstering without looking is a useful skill that people should practice, that could be useful in some limited situations. But I think it is overemphasized sometimes.
Edit- FYI, I recently took a class from Kyle Defoor, who advocated looking at the holster as you holster, for pretty much the same reasons as above.