When to divide rhubarb

The general rule seems to be divide every 5 years or so.  Mine was fab this year, but huge and congested.  It had quite a lot of thinner stalks and the crown is large and has pushed itself way up out of the soil. 

How do I know whether I should divide it?  If I don't divide it, might the crop be less next year? 

Or can I leave it for another year, and possible chop a bud off to start a second plant this year?

Thanks Trillium, was guessing that was the case.  Will get dividing, you can't have too much rhubarb.