It's fine to do this, but some women spot or have bleeding like a period despite trying to skip. Other then this "risk", however, it's fine, and for many women it works wonderfully.
One thing to consider: are you on a triphasic or monophasic pill? With triphasic pills the amount of hormones in the pills varies by week, whereas with monophasic the amount of hormones is constant for all twenty-one pills. Many women have trouble skipping using triphasic pills because of the changes in amount of hormone throughout the pill cycle.
Im actually using the nuva ring, where I would leave it in for the extra week instead of a week without, Im not sure if that makes any difference.
Nuvaring is only guarenteed effective against pregnancy protection for 3 weeks at a time. By keeping Nuvaring in longer than the recommended 3 weeks, you're opening yourself to a higher risk of pregnancy.