Before we can answer that question we need to face the fact that there are so many different kinds of hardwood floors. The most popular type of hardwood floor is engineered hardwood floors. Most types of engineered hardwood floors are glued together using special types of glue and what will happen is the steam from the steam cleaner is going to affect the bond that holds the wood together thereby damaging the joint. The steam is going to get between two ends of the wood and cause the word to separate.
The next type of hardwood floor is solid hardwood floors. There is a mixed opinion about whether or not you can use a steam cleaner on this type of hardwood floor however, it is generally safe provided that it is solid wood and sealed. If you have a type of hardwood floor that does not have a seal on it then it is not safe. To find out if your hardwood floor has a seal on it simply put a little bit of oil on it and if it absorbs into the wood then it does not have a seal. If however, the oil does not absorb into the wood then it does have a seal and its safe to use a steam cleaner.
When it comes to laminate floors it is highly advised to keep your steam cleaner as far away from your floor as possible. Laminate floors are manufactured using a type of particle board that does not react well to moisture. If you were to steam clean your laminate wood floors what would end up happening is the wood would actually warp and bend. The damage that would be caused would be irreversible.
The next type of wood flooring is painted wood flooring. This is also a risk because what can end up happening is the steam can penetrate the paint and cause bubbles thereby allowing steam to damage the wood. Rather not use a steam mop on painted wood floors.
The best type of floors to use a steam cleaner on are tile floors. They are solid enough to withstand the heat that a steam cleaner generates. Just remember that marble floors are not in the same category as tile floors and you must not use a steam cleaner on marble floors. The steam will penetrate the pores and rupture the marble.
For floor types that are not safe to use a steam cleaner on always use a microfiber flat mop found at http://www.amazon.com/. These types of mops are purposely built for delicate types of floors. They do not leave excess moisture on the floor which can damage most wood floors.