[Tim Maloney]"The tiffen makes sense as is can reside there forever! "
Although I'm still waiting for the two Tiffen T1 filters I ordered I can report from others that leaving the T1 on all the time is not recommended. Users are reporting a myriad of problems depending on what your subject matter is and what your lighting conditions are.
Some of the issues are:
green caste to some of the colours in the picture
light loss is around half a stop
some users are reporting a general "muddy" look to the picture
If you get lens flare there's a good chance it will have green in it
To summarize, the T1 affects other colours than black differently depending on light source. The recommendation seems to be leave the filter off unless you have a significant amount of dark clothing in your shot, and then only if the lighting causes the clothing to turn dark red or brown.
I use the tiffen, but only when I have to - I do not leave it on. Works great. Have not noticed any color artifacts as mentioned, but it only makes sense to keep the number of air-glass surfaces to a minimum for sharpness. Aside: I actually left it off intentionally once because the "brown" black helped separate the subject from the black background when I didn't have a backlight. Go figure.