Depending on the gun...
You would less likely damage the extractor by dropping the round in the chamber manually, then closing the slide over the chambered round. That way the round you are rechambering will never suffer setback of the bullet. It's also easier to load a full mag +1 in the chamber this way.
I KNOW, I KNOW, there are many many internet experts out there that will tell you not to do this, but my gun, Taurus Millenium Pro .45 has no problems at all with this. I would say it is NO problem with a gun that has a spring that holds tension on the extractor, it MIGHT be a problem with a gun that just uses the flex on the extractor to maintain tension.
This is what I do:
Drop the round in the chamber.
Ride the slide forward by hand with just enough force for the extractor to engage the round.
Do a brass check - pull the slide back enough to observe proper partial extraction of the round from the chamber.
Push the slide forward, ensuring it goes into full battery by observing that the back of the slide is even with the top of the frame.
Insert full magazine.
I have had ZERO problems with this procedure after performing it hundreds of times.