It actually is frowned upon, but not for reasons you would expect.
The main reason is that many other people read these forums besides the helpers and the helped. When people search for problems, it is often this site that pops up. Those readers need to be able to see that their problem is the same, see the steps tried, and then see the SOLUTION. If you solve it for a member over skype, thats great for them, but others need to be able to see it.
If you do help someone over sykpe, make sure that either you or they post the successful solution on the forums for the rest to look at.