I don't believe in filters for protection on lenses, with the possible exception of a situation where I know dangerous chemicals might be thrown at the front element of the lens (e.g. in a chemical factory). (See FAQ on volcanos).
Filters always rob a bit of light (and in some cases, a lot of light), increase flare tendencies, and add to your equipment cost. To date, I've not seen a single controlled study that shows that filters actually offer real protection. Indeed, some of us believe that the opposite is sometimes true with cheap filters: if a cheap glass filter shatters the shards tend to scratch the front element of the lens.
The real reason why every store salesman asks you if you'd like a protective filter with that lens you just bought is because it increases the store's profit margin. Considerably. Consider a US$1000 lens and US$50 filter. The store will make US$150 on the lens, US$25 or more on the filter. Thus, even though the filter added only 5% to the cost of your lens, it added 17%+ to the dealer's profit. The only thing that's more lucrative is if they can sell you an extended warranty with that lens (in the US, a Nikkor already comes with a 4-year extension to the warranty, by the way).
See my article on filters.
