Some do, some don't. It sort of depends on a lot of things, such as whether or not the classes will help you in your current position, or help you to advance to the next level, within the company. Many are not interested in helping you to better yourself so you can leave them and go elsewhere.
A lot depends on your field of work. Tech and science/medical are more likely to offer tuition assistance, as are schools and gov't employers. Some will require that you get certain grades, and will reimburse a percentage of the tuition based on the grades you get. Try to research company benefits on the website.
It's good to see that you are thinking about tuition assistance. School is expensive, and any help is welcome!