It is recommended that the sun should
always be behind you when taking pictures and that photographing on
cloudy days was recommended at all. Even if this were technically
possible, the insufficient brightness and demand on ISO sensitivity
will make it a hard time to produce a good photo.
Though the technology on camera sensors
has been improving steadily over the years, some DSLR can still perform
really well with 1600 ISO or even 32000 ISO and thus speed have
increased without impairing much on picture quality. Photographs that
would have been impossible to take a couple of decades ago, at least
without at a time exposure, are now so easy that everyone can take a
cloudy day in his stride. And with more and more type of noise
reduction software available nowadays, it gives no excuse not to take a
photo in cloudy day anymore.
That is not to say that ninety-nine times
out of a hundred one wouldn't prefer the sun to be visible. Under a
dense, overcast sky the lighting is always flat and everything looks
gray. As a result a given scene offers much less contrast than it would
if the sun were out.
At the same time, by making the most of
such situations you can take unusual and often hauntingly beautiful
pictures. For instance, interesting pictures can be made when you find
a dark subject that stands out sharply from the background and creates
an interesting pattern. The reverse is also true-snowflakes, for
instance. I have found that snow photographed in black and white on a
dark overcast day can produce much more dramatic results than a similar
scene shot in bright sunlight. I always used to shoot snow scenes in
bright sunlight; now I am prepared to make the most of whatever the
weather man predicts.