Should the audio output change when plugging in headphones?

I was wondering if the audio output within the sound preferences should change automatically when plugging in headphones in the front headphone jack port? I have a Mac Pro Dual-Core 2.66GHz that I use at work and noticed when I plug-in my headphones the sound output changes to headphones in the sound preferences, but my home computer which is a 2010 Mac Pro 8-Core 2.25Ghz and I have to manually change the sound preferences to headphones when plugging them into the front jack. The only difference between the two computers is I have a subwoofer and satellite speaker system hooked up in the back audio jack on my home computer. Would this prevent the sound from switching over to headphones in the sound preferences?

Thanks,

Darren