Originally Posted by TugaDude

There are a number of reasons why someone might want to flip their back wheel. One would be if they wanted to change from fixed to freewheel or vice-versa. Reasons to do this might include wanting to coast on a leisurely ride or something like that.
Another reason might be to change sprocket size. You can have different sized cogs on the rear wheel and change from one to the other in order to climb steeper hills, etc.

As far as things you need to know, yes, know that the track fork end will limit your ability to position the rear wheel and that impacts the length of chain required. In other words, if your chain is tight on a larger cog and you flip it to a smaller cog, there might not be enough room in the fork end for you to take enough slack out of the chain. It also depends upon the difference in size between the two cogs. A fork end with a long slot is helpful. I don't know about yours. Know also that there are "eccentric" bottom brackets available that can help with the slack issue.

Finally, how often do I do it? I can't remember the last time, so that answers your question.