Depending on what you are measuring, you may be able to use the ratiometric method for your a/d in which case the reference accuracy is unimportant as your measured value is a ratio of the reference. The ratiometric method is fine if you are measuring resistors, if you are measuring external voltages etc, you will need a stable reference. The fact that your 5v rail varies depending on the load suggests you need some form of better regulation.

Zeners are not considered good references as they have a high initial tolerance of about +/-10% and they drift with temperature. Go to the web and pull up a datasheet on a zener and compare the specs with a reference ic from maxim,analog devices etc. Also, low voltage zeners have a poor 'knee' characteristic so the zener voltage depends greatly on the current you're putting through it.