I had a gun store employee go off on a rant about this when my wife and I were shopping for a gun for her.
He said, "It's not a slide RELEASE, it's a slide LOCK and you don't chamber a round by using that there lever, you ALWAYS rack the slide to chamber a new round..."
I said I had just watched several competitors the night before loading fresh mags, then releasing the slide with their support hand's thumb on the lever, and it was working well for them. They were 1911 (single stack) shooters.
His response was to grumble, "Different school of thought."
By the way, Mr. Gun Store Employee, the part is called a slide STOP on most semi-auto pistols, and that single part serves as both a slide "lock" and a slide release.
All this was after loudly scolding my sweet wife for daring to touch the trigger on the XDm that had its slide locked back, just to check for finger reach with her small hand, whilst the male neanderthal potential buyers were pulling triggers left and right. And I know from experience that this particular gun store has let me do the same, many times, sometimes acting bemused that I asked permission to pull triggers.
OK now for my own take on it. I've had a couple of failures to feed when I used the lever to release the slide after loading a fresh mag. But I've never had a failure to feed off of a fresh mag when racking the slide all the way back and then releasing it as my hand continued in a backward motion, supporting the principle that with this technique you're using the full potential of the recoil spring to help chamber a round. On the other hand, I think it's faster to release with the lever as the support hand is brought up. So like many are saying, do what works for you.