Soupy has raised both the pros and the cons in his above reply.
For ultimate quality don't add any filters but make sure that you are happy and can afford to replace/repair the lens should the glass become damaged at all.
A UV or Skylight filter is recommended as a cheaper solution to replacing the lens in the event of damage. The filter takes the damage and thus its cheaper to replace that rather than the lens. The trade of is loss of quality. The amount depends on how good the filter is and that, like everything is related directly (in most cases) to cost as well
All filters will reduce the quality but by how much and what is acceptable will depend on the filter itself and the photographers demand on his/her equipment.
So there is no straight forward answer and no right or wrongs
