You are OK to workout if: you have a head cold with no fever, or if you think it's more allergy related. A good rule of thumb to go by, if your symptoms are from the neck up, you should be ok. Just be sure to take it easy, and listen to your body.
You should probably stay home, (and see a Dr.) if: you are contagious, coughing, running a fever, nauseous, vomiting, severely achy, fatigued or have any sort of chest congestion.
Although sometimes working out when you have a slight cold can make you feel better and boost your energy, some people are convinced that it's always best to "sweat out the sickness." This can be very dangerous. If you put too much stress on your immune system it may prolong your illness and make it harder for your body to recuperate. It should be known that if you overdo it you will most likely pay for it. Breaking your routine may be difficult, but it will be worth it if it speeds up your recovery instead of prolongs it.