There are, in fact, multiple kinds of viewing filters--- perhaps Fred Picker never understood that (just as he never quite unterstood much but he had a lot of enthusiasm and was effective at selling snake oils). Ansel Adams seemed to go for the Wratten#90 but there are really better filters around.
Originally Posted by David Brown
Are they worth while? I learned to use them back in my cine days. They are not, as commonly held here, for judging B&W but to judge lighting contrast. The light levels are attenuated so one can get a better indication of what kind of shadow detail might be possible and if fill lights are called for. One would use a set of other filters to judge the tonal separation. Together, I've found, one can better respond to conditions.
I still sometimes carry my old DuPont viewing filter. Its colour is something between the panchromatic viewing glasses sold by Harrison or Tiffen and the yellow/amber glasses. If I want I then hold a yellow, green or red filter (if I'm considering one) in from of my viewing glass.