The top of a table saw should always be clean. Any build up of rust or other materials can not only look bad, but it can be dangerous. The last thing you want to do is have your saw blade traveling at top speed, start pushing the wood through, and then get caught up on a knot or something else due to rust or another foreign object. Just a little bit of maintenance can keep you table top clean.
To start, you can apply a light coat of wax to your table saw top about once a month. This should help keep it clean and prevent any rust from forming. If you do have spots with light rust forming, you can use a steel wool pad and some elbow grease to take that rust away. Again, once you have removed the rust, apply a small coat of wax. For a really bad top, spray a little WD40 and use some sandpaper to cut away the rust, then move down to steel wool. Once you have the rust off, you will want to clean it up with a small amount of soap and water to remove the WD40. Then when you apply the wax, it will stick.
Remember to take some time and apply a small amount of wax each month to help keep you table saw top clean and rust free for years to come.