It definitely isn't a no no mate!
Stillwaters can be really good in the winter, posasibly more so now our winters are warmer!
It is true that some waters are certainly more difficult as the fish's metabolism slows down and as such they eat less and possibly some waters might not fish at all well.
However all waters are different, just adapt your approach, feed a little less, and depending what you are fishing for possibly reduce your hook/line size!
As a general rule, look for warm cloudy days with a bit of rain, and if you can afford to be choosy sometimes bright frosty days are often the most difficult.
What sort of water is it? What species are you after?