First of all, most of these posts presume a somewhat "pristine" environment will exist in a post-shooting situation.
Murphy's law of Combat #6: No plan survives contact with the enemy.
All of the advice above goes out the window when you're forced to shoot Danny Dirtbag in the 7-11 parking lot because he came at you with a knife. People pulling into the parking lot; people driving off; pedestrians coming & going, etc. Not to mention you might draw something of a crowd before LE arrives.
Some of the advice also turns to vapor if the subject is still alive with an incapacitating wound. He may be issuing threats to you, complaining of his pain or whatever.
And it's different again if the perp has a gunpoint ephiany about bringing his knife to a gunfight and surrenders. (Now your choice is hold 'em or cut him loose.)
There are techniques to control a gathering crowd. The more you appear to be "in charge" the better. Keeping the crowd back so your perp's weapon doesn't disappear and/or keeping "Good Sam" back away from a potentially still-armed subject are only two worries.
I'm of the opinion that I'd prefer to holster my weapon before police arrive but only if it's safe to do so. This way I can wave two empty hands in the air. Regardless of holding the gun or not, I want them to see me as "flagging them down" or waving to them.
Once they arrive, they command. I'll do what they say... although I'm inclined to lay down my gun instead of dropping it.
Remember, police are taught to "secure the scene" of a crime. That means putting into handcuffs anyone suspected of violent acts and securing all weapons.
BillCA in CA (Unfortunately)