The rear main oil seal or crankshaft seals is one of the most difficult to replace. However, they often fail, and when they do, your car will be marking it's territory with little oil drops wherever you park. Apart from making a mess, it can also cause oil to come in contact with the clutch surface. Replace yours and keep your oil in your engine.
The perfect time to replace this seal is when you are doing a clutch replacement job. You already have everything apart, so you might as well add this piece even if it hasn't failed yet as a bit of preventative maintenance.