Generally this is a healthy shine, but many people interpret is as looking wet or greasy at first, just because we're so used to the look of dried out, overwashed hair.

If you use a shampoo (as distinguished from a low-poo) after you have been CG for a while, you may or may not notice some deterioration in your hair condition. Provided it's a single incident, and you go back to CGing, it may not make a lasting difference in the condition of your hair.
Peace,
Morgan

Baby Fine 3B, low porosity, normal density and elasticity
CGing since July 2008