Just to make sure you are clear:
Don't allow the slide to slam shut on an empty chamber.
However, when chambering a round, allowing the slide to go forward at full velocity is preferred.
It is also advisable to train without using the slide release. To release a locked open slide, grasp the slide firmly with the gun pointed (as always) in a safe direction. Pull the slide fully to the rear, and release, allowing the slide to go forward under spring tension stripping a round from the mag and chambering it.
There are two reasons this is preferred. First, it is the same manual of arms for 99% of the auto-loading pistols on the market. Second, it more closely approximates what happens when firing the weapon, giving more consistency to each round (including the first).