Filters and do we need them with digital photography?
I don't believe you have to have a filter on your lens all the time, eg; for protection. In fact I only regularly use two or three types of filters. A circular polarizing filter and circular ND filters and sometimes I will use a split graduated ND filter. The only time I put a clear type filter on the lens is if I'm working at the beach with salt spray or sand blowing.
My recommendations for filters is buy the very best you can afford and in the size of the biggest lens you think you will buy....in most cases this would be 77mm. The reason? Well you only have to buy the one filter to fit all lenses when used with step-up rings (imagine the savings there if you own two or three lenses)...Put the money you save towards the best quality filter you can afford.
Get rid of the plastic cases that filters are sold in...these cases tend to "out gas" (the foggy film that you see on filters when left in their case for any period of time) and buy yourself some "stack-caps" these are cheap and protect your valuable filters and don't "out gas" PLUS if you follow my recommendation and buy all one size of filters, you screw all your filters together...as one and fit the caps. Overall a far neater and functional solution.
Just some helpful thoughts!