Cannot mix through headphones? Bull. I've mixed through headphones just fine.
Obviously monitors are better, but only if you have ones that give you any sort of accurate representation of your sound, and those tend to be a lot pricier than monitor headphones.
I started a similar thread on cakewalk.com's forums: Computer speakers
You CAN... but the mix won't translate. Do what you want man, you don't have to take my word for it. Just telling you how it is.