Subwoofer

I have read the same over and over as well. If you have a good set of studio monitors then that's basicially all you ever need.
There's good and bad about using a sub. 
A subwoofer can result in you not mixing ENOUGH bass into your mixes. 
But then I use mine on occation to pick out low fequency content my NSM 10 will miss. 
I have a Sony sub on a aux send from a Mackie Mix 8 I use for control room extras, like headphones and the sub. The Mix 8 gets my 3/4 output from my interface. 
I also have a small PA hooked up to proof my mixes I use for backing tracks. 
 
I would think a sub would be handy for proofing mixes that are headed to a system that uses a sub. 
But the advice of many "who know best" is to only turn it on when you need it. Don't have it on all the time. 
I got my Sony Sub from Goodwill for $18. It seems to be part of a better quality home theater system. 

A sub used correctly should simply extend the frequency response of your existing system downwards. This requires careful matching at the crossover point so you get a flat frequency response as you transition from the mains to the sub.
 
The problem most people have is an overlap at the low end of the mains which gives an artificial boost in the 80 - 150 hz range.  Kick, bass, left hand of piano, etc, all sound to loud.  You need a good HPF on the mains to remove the low frequencies that you're using the sub for.
 

Many subs come with a handy footswitch for bypassing them. If you do get one, make sure it has that feature.
 
Whether a sub will help or hurt depends a lot on your room. If it's square or cubic and lacks bass traps, a sub will likely complicate things and may not be worth the bother.
 
Even if you have a large treated room of favorable HxWxL ratios, the sub is still be an optional luxury if your main speakers already extend down to around 45-50 Hz (-3dB). There just isn't much of musical importance happening below that, mostly just unwanted rumble. If somebody else is mixing and mastering, then you needn't care about that. A spectrum analyzer will tell you what you need to know. 
 
If you haven't done any objective measurements on your monitoring environment, now's the time to do that, before you blow any money on a sub. Find out exactly where your speakers wimp out, and where your low-frequency room resonances are. That information will tell you if a sub is needed, optional, or undesirable.
 
My own situation (probably irrelevant to most folks) is this: I have a medium-quality 4,500 cu. ft. room with just-OK dimensions and inadequate low-frequency acoustic treatments. IOW, it barely qualifies as a suitable environment for a subwoofer. I could do just fine without it, in terms of mixing and mastering. However, I also listen to movies and video games through these speakers, where sonic accuracy isn't as important as just shaking my chair - and the sub's very nice for that.

Lots of great info here guys. Thanks. Once I finally get this room setup the way I want it to be, maybe I should finally test the room out like bitflipper said and get an actual look at if things are the way they should be, and if a subwoofer would be good/bad. My room isn't really treated in any meaningful way since I don't do any live recording (all synths and direct input guitar), and I usually do the majority of my mixing in my headphones.
 
In the meantime I'll just stick with what I've got, and like I said in my other response, probably make more of a habit out of giving a quick test on my home theater system to TV-intended stuff.
 
Thanks :-)