1. Consistently shampooing with the same shampoo can over time cause build-up of residue in the hair. This can cause the hair to be weighed down or dull. To easily avoid this, change up your shampoo every now and then. An easy tip is to use a clarifying shampoo once a week.
2. It is most helpful when styling your hair if you have previously used a shampoo to complement your style. For instance, if you have naturally wavy or curly hair and are going to let your hair air-dry or diffuse it, you should use a shampoo to enhance your natural curl. If you want to style your hair for a fancy event or do a creative updo, then some sort of volumizing shampoo would be best.
3. If you are consistently using just a volumizing shampoo (or curly, or smoothing), your hair can become accustomed to it and it will stop working as well. This is why it is best to have several shampoos that you like and rotate them accordingly.