One important fact to check before placing any rug over your hardwood flooring is to make sure that it is non-staining. Non-staining means that the dyes in the rug will not bleed through onto the surface that they are placed upon should they get wet. Generally the manufacturer will put a warning on its product if the dyes will fade through due to moisture. Be sure to read the label on any rug before placing it on your hardwood floors.
You also want to make sure that the backing on the rug is not rough. Hardwood flooring is susceptible to scratching and any rough backing could damage your floors.
When placing throw or area rugs in foyers, entryways and hallways or high traffic areas make sure that you regularly clean underneath these rugs. Dirt and debris under a rug that is in a high traffic area can scratch a hardwood floor. Also, during inclement weather such as rain or snow be sure to remove and dry these rugs because moisture is detrimental to your hardwood flooring.
The floor's finish will age so it is important to move the area rugs occasionally so that the color of your hardwood flooring remains uniform. Sunlight will also affect the coloring of your floors which is another reason to move your area rugs on occasion.
Some area rugs are backed with vinyl or rubber. Check with the manufacturer of your hardwood floors to make sure that this backing will not react with your floor's finish because some can cause the finish to become dull or discolored.