I hate to say it but Massive Master is 100% right on this one.
That said, I have a sub in my homestudio, even though my calibration skills are less than adequate. The important thing is to be aware that you are mixing in an environment that is not ideal by any means. One way to make sure (my preferred method) your mixes are good is to have reference mixes while you are working on a mix of your own.
I'll usually have 3-4 reference mixes that might approximate bass levels & overall sound Im trying to accomplish in a song or a song section. Obviously this isnt foolproof, and you end up relying quite a lot on your ears (although frequency analyzers tend to help).
The reason I've decided to take the particular risk of adding yet another variable to a uncalibrated setup is this: Studio Monitors cannot reproduce bass properly, at least to my ears, especially below 100Hz, which is where all the sub-bass frequencies lie.
For example, recently I was mixing a song, and decided to use a Red Hot Chili Peppers song as a reference for the kick drum sound. Due to the limited bass reproduction of my studio monitors I always ended up boosting frequencies between 100 & 250 Hz, effectively making the bass stand out in my system, but really just making it muddy overall. I soloed my reference track and turned on my sub, low and behold the kick bass was thumping like mad. when I placed a frequency analyzer on the track I realized the bass thump of the kick was at around 60-70Hz.
This was the first time I realized you kinda need subs or some very huge monitors like a lot of professional studios have. Otherwise you're just shooting in the dark with the lower bass frequencies. Granted reference mixes are a big part of it, since without them you are shooting in the dark as well, unless you follow MM's advice and do some meticulous calibrating of your room and equipment.
Regarding placement, I have mine below my desk a bit off center relative to my speakers (due to lack of space) and it seems to do the job just fine.
anybody feel free to provide alternate advice, since I am not an expert by any means