I have not had a problem dropping 1 in the chamber then closing the slide.
After doing it that way I always check to insure my slide if fully closed and also insure the loaded chamber indicator is in the correct position.
I may be doing this wrong in someone else's eye's or mind but it works for me.
I have a buddy that is going through the local police academy. They are in the pistol traning portion of the academy and they are shooting Gen4 Glock 17's. Since I owned one I asked him if he wanted to come out with me and practice.... Well he loaded up the mag to the max, inserted it, then began to drop one in the chamber. Glad I was watching him because I stopped him right away and told him no, NEVER do that unless it is 100% absolutely necessary. He looked at me and said "this is what we do in the academy......" I was dumbfounded.... I told him while you use my guns, you do it my way. Insert mag, rack slide, drop mag, top off and re-insert.