Originally Posted by MistWolf

It's your rifle, it's your training.

However, practicing mag changes with the bolt locked back ensures you practice pressing the the bolt release enough to release the bolt every time. The problem with slapping the bolt release is that it's easy to cup your hand over the release and not hit it hard enough to release the bolt. Perhaps with more repetitions, you'll get down to where you don't cup your hand. But pressing it with your thumb takes fewer reps and doesn't fall apart as much when you're fatigued or cold or using your AR in inclement weather. It ensures your hand is positively indexed to make it easier to find the mag release in the dark or while your eyes are scanning down range.

Whichever method you use, actually locking the bolt back and releasing it during practice ensures you're getting it right.