Scientific American Mind on Crying
The takeaway? In the lab, people seem to feel worse on average after crying. But the lab isn't anything like the real world (say, your own bedroom), so take that with a grain of salt. In real life, people report feeling better. But real life isn't like the lab, where they can make much more accurate assessments.
More interesting details:
Criers who received social support during their crying episode were more likely to report mood benefits than were criers who did not report receiving social support. Likewise, mood benefits were more likely when the precipitating events of a crying episode had been resolved than they were when events were unresolved. Finally, criers who reported experiencing negative social emotions like shame and embarrassment were less likely to report mood benefits.Some people do report feeling worse after crying, in the real world, too. So I guess forget about folk wisdom for now, and make your own decisions about crying :)