It is definitely an acceptable practice. As a matter of fact I would always encourage colleagues and co-workers to watch certain forums. Not just use them when a problem arises and you quickly need an answer but as a routine part of the job!
I've often found that just by looking on new postings you can always learn aspects of tools you're already using, new ways how to accomplish things and new featured that you wouldn't have discovered otherwise. In addition you often get a sense of what people are interested in, what their motivations are and how things are done outside your office.
So all these arguments lead to the conclusion that it is time very well spent. Anyone not accepting this, or even forcing me to discontinue watching certain forums (or blogs and other sources of information) completely disqualifies himself.
And yes, I agree with another poster on this site: If that ever happens and your boss tells you not to look into forums any more it's time to move on and find another job.