It is never recommended to put an area rug with a rubber backing onto hardwood flooring. Most manufacturers state that you should only put a rug that is "breathable" on top of a wood floor.

Sometime moisture will get under the rug and if it cant breathe that moisture will continue to sit on top of the hardwood floor causing damage.

Thank you mpeterson, you answered my question.

Polyester latex backing, OTOH, will not harm wooden floors. When in doubt, use a non-backed rug, and a padding that is guaranteed for all types of flooring. You can find them in various levels of quality at carpet stores, flooring stores, or the Bed Bath Beyond and JCPenney type of stores.