Extractors can be forced around a cartridge rim, there's no denying that, but it's bad on the extractor to do that...probably some guns more than others. Some extractors can break despite being incredibly harder than the brass case they're slammed against. The movement of an extractor is not intended to be pushed out and around a rim and it's been told to me that it will mess with the mechanisms behind the extractor as well.
Basically, as posts 2 and 3 describes, the natural or intended way to load a round in the chamber is from a magazine so that the movement of the slide going into battery can guide a round up into the extractor's grasp as it enters the chamber. I wouldn't slam the slide on my 1911 around a chambered round I put in because there's no natural resistance and it's pretty much the same as slamming the slide on an empty chamber.
Can you get by with slamming your extractor around the rim of your manually inserted cartridge? Probably. Are extractors easy to replace, sure. Are you risking a reduction in shell extraction consistency? Possibly. Would you like to run the risk of FTEs when your life is on the line? I wouldn't.
I was faced with this debate and I err'd on the side of caution because you can't know from the testimony of others how your gun will fare. Also, it's not a big deal to load that +1 via the mag, then top off the mag and do your loaded check before you leave the house.