I have a 96 F150, and when I turn on the lights at night AND I turn on the blower motor, the needle swings way down the left side of the scale, almost into the red area. As soon as the rpm's increase a little, then it is fine. I replaced the Alternator last year and it still does it. Is it the battery? I wouldn't think so.
Rich

 

I would say it's pretty normal. The voltage of the battery decreases as you put more and more load on it. If you run the alternator at high rpm's it helps the battery by recharging it, hence the voltage increase. Seems perfectly fine to me...

 

With lights, blower, and turn signals or wipers the guage can drop. My '95 stayed closer to the middle though. With my pulleys it sometimes drops to around 10 volts at idle with accessories running.

Seems like they vary. I have a friend and his drops at least as much as mine, but he has stock pulleys.

I wouldn't worry unless its getting out of the normal range. As long as the charge comes back up once you hit the gas you should be OK. Could just be a slightly lower idle speed.