Well, U should ALWAYS clean and reoil a new weapon before U shoot it the first time. Sometimes there is not enough oil in certain places, or even packing grease.
And, U really only need to leave the mags loaded a while IF it is a mag that gives you a hard time loading the last 1 or 2 rounds. If you have no problems, then its not an issue.
Also, it will take time to learn not to over oil and not to underoil. I use a toothbrush to put oil on the rails on the frame. And, even if I don't shoot the gun, I will take the slide off about 1x a month and reoil the rails with a toothbrush again (go buy a $1 cheap one). Oil evaporates over time, and you want that area to stay oiled (but not to where it is dripping off the gun).