The Ultra Pro Situation
Without a doubt, the most important factor in monitoring is the acoustics of the room and the placement of your monitors in that room. (Read my article about moving your studio monitors). The major label big boy recording engineers have rooms that cost as much as a house. The acoustics in these rooms are amazingly even and this allows for the most accurate monitoring possible. The big boys all seam to use different studio monitors because their tastes and mixing styles are much different from recording engineer to recording engineer. I will put up money to back my my claim. No big boy mixer is going to pick mixing a record in a shitty building with their own monitors when they could mix in a great mixing room with some other monitors. In other words, the room is much more important than the monitors/speakers.

The Ultra Broke Situation
You are recording in your bedroom. You bought a few cheap condenser mics and are going to town on your record. Can you get great quality with your home stereo system? There is no reason that you can't to be honest. Depending on the speakers you use , the room you use, and the way you hear things, there are lot of factors that come into play and the speakers are just one of them. I would consider it much more important to use speakers that you know in a familiar setting than to just run out and buy new studio monitors. If you are just starting a studio and haven't done much mixing, you are in for a learning curve from hell. There are some many things that you must learn to get your mixes to translate well to the outside world regardless of you monitoring situation that I'm not sure if it matters what you are mixing on when you get started. I started with Mackie HR824s which are by no means the greatest studio monitor of all time, but I've been told by several big boys that they should be sufficient for cranking out kick ass records (assuming the room is great). Well, let's just say that the sound coming out of my studio has changed drastically over the years and my monitors have stayed the same. In other words, I could have been mixing on a phonograph and it probably wouldn't have made that much difference.

One Great Point
The most popular studio monitor of all time started out as a bookshelf speaker. I'm talking about the Yamaha NS-10. These are the black speakers you see with the white woofers. They have been used in just about every big studio in the country. Their story is funny because they some of the worst sounding speakers ever made. That's probably why they work well as studio monitors. This just goes to show that home stereo speakers can work out just fine for you.

Problems To Consider With Stereo Speakers

Studio Monitors Were Designed For Mixing
Generally studio monitors are voiced in a way to can make your mixes sound bad...seriously. Home stereo speakers are voiced to make your music sound good. This usually means the mids are scooped quite a bit in a stereo system vs a studio monitoring system, but not always. They make studio monitors for a reason. Studio monitors are designed to make your mixes sound better than standard speaker systems.

Think Long Term About Monitoring
The toughest part for me when it comes to mixing is knowing the monitors and trusting that the sound I'm hearing is indeed accurate. You have to learn your system very well to mix well. There is nothing wrong with starting out on stereo speakers, but if you someday intend to get studio monitors, you may be better off getting them as soon as possible. The sooner you start using your monitors, the quicker you will understand them.

Conclusion
There is no reason that you can't start out using your home stereo speakers to mix your home recordings. Check your mixes on other systems repeatedly and you should do fine. Just remember that if you have plans of being a great mixer, you'll want a nice set of speakers in a great sounding room.