I use a lot of telephoto lenses, similar to your 150-600mm.... I almost never need to use any filter on those.
You didn't say what lens you use with the filters you already have, but I would guess it's an EF-S 18-55mm, with a 58mm diameter filter thread. For that type of lens, to me the UV would be a very useful filter for landscape photos, to deepen the blue of the sky, reduce reflections, make foliage more colorful, etc. It's not a filter I use all the time, but C-Pol are by far my most used.
In fact, there are many times when a C-Pol shouldn't be used. If you are trying to photograph a rainbow, a C-Pol filter will make it disappear! There are also times when a reflection is part of the image and the C-Pol would just ruin it. A C-Pol reduces the amount of light passing through to my sensor by 2X to 4X (approx.), so there are also many times when using one would cause too slow a shutter speed or too high an ISO to be necessary. This is one reason I rarely use one with telephotos, where shutter speeds can be pretty critical.
Finally, a C-Pol is most effective at 90 degrees from the light source, with its effect gradually tapering off as the light source is closer to 0 or 180 degrees... So, for example, a C-Pol serves no purpose when directly shooting a sunset or a sunrise. But if pointing the other direction, or if the sun is higher up in the sky, it can be useful.
So, even though a C-Pol is probably the most useful filter for digital photography, and easily the one I use most, I probably only use one 10 or 20% of the time.
I have UV filters for my lenses, which I only install when necessary such as out shooting in a sandstorm! I use them at the seashore, because "salt air" on lenses is difficult to clean off and I'd rather wash a filter than the front element of my lens. UV are primarily used for "protection", which is pretty silly in most instances... after all how much protection can a thin piece of glass provide? Plus lens elements are a lot tougher than people think, while lens caps and lens hoods both do a really good job protecting lenses. UV have little other use today with digital. There can be rare instances in scenic shots where there's a distant blue haze that the filter can reduce a little. They are pretty low priority, but if you find yourself shooting in certain situations with it, you might want a UV for that 150-600mm.
Other than that, filters useful for digital are pretty highly specialized. Neutral density filters are used to allow really long shutter speeds or really large apertures to be used in brighter light conditions, where the exposure setting range of camera and lens aren't sufficient. For example, still photos of running water are sometimes shot at slow shutter speeds to make the water movement appear "creamy". A 3-stop or 5-stop or even darker ND filter can be used for that. Between your two lenses, I'd probably never use the 150-600mm, but might take this kind of shot with the 18-55mm. So I might want one or two ND filters for that lens. This would only be important if planning to do that type of shot, though. (Or, the other example, wanting to use a large aperture like f/1.4 or f/1.2 out in bright conditions... which doesn't apply here because neither of your lenses have a large aperture like that.)
For portraiture in particular, I have a few "black mist", "black spatter" and "black mesh" filters that are used to reduce fine detail and make skin look smoother. These are also what I'd call highly specialized filters. I only have them in the size needed for my primary portrait lenses and don't use the filters very often.
Back in the days of film we had to carry warming, cooling, color correction and conversion filters. These are not needed with digital because we can simply set a Custom White Balance in the camera. And shooting digital, nearly all other "filter effects" can be accomplished in image post-processing... on the computer, with image editing software.
So unless you have need for some of those specialized filters, if it were me the only filter I might add to your kit would be a 95mm UV for very occasional use "protecting" that 150-600mm in certain situations. Other than that, it sounds to me like you are pretty well set.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2) some other cameras, various lenses & accessories
FLICKR & ZENFOLIO
