killerman,
Changing up your routine is based on the assumption that you "shock" your muscles to encourage growth. The idea being that it "confuses" your muscles so they never know what's going on and cannot get accustomed to the exercise. This is the theory anyway. I've never found any research to support it. If you think about it, it doesn't really make much sense. Your muscles don't remember what exercise you were doing last time. They really only respond to one thing: overload.
If every time you work out you overload your muscles by either doing extra reps, extra sets, or extra weight, then I guarantee you will see results. I've pretty much done the same workout for about 2 years now and I can safely say that I've continued to see results.
Plateaus really only happen for one of two reasons: either psychological or it's an indication that something needs work (either your technique is incorrect, you're not eating enough or eating right, etc.). Changing up your workout may help if it's a psychological thing, but it is not a requirement to make continuous progress.
The only time changing up your workout becomes necessary is if your goal changes. For example, say you went from wanting to build muscle to needing to cut and train for an amateur fight.
Some people do get bored doing the same workout, however, so changing it up can certainly help there. My recommendation is to change it up about every 2-3 months. This gives you some time to get in a bit of a routine so you can monitor progress. It's totally up to you. You'll get results either way, but don't start thinking that changing it up is a necessity.
-John
