I think perhaps a good rule of thumb with respect to filters
for use on digital cameras is "Use only filters which
enhance the clarity and tonal range of the captured image.
Most all special effects can be simulated in post
processing." If you make an image with a given filter,
you're pretty much stuck with that effect, as it
is very hard to undo the effect in post processing. What
I would aim for is the sharpest image with the best tonal
range as a starting point, and then work from there in
post processing. This gives me the freedom to change my
mind about effects after the fact.