"off" has the meaning of "gone" or "lifted up".
Hence when you use "wipe off the table", it means "whatever on the table you remove it"
When you use "wipe the table", it just means "the table is wiped, whether everything on the table is gone (lifted up) is UNKNOWN".
Therefore, if there is nothing on the table, you can use both "wipe the table" and "wipe off the table". If something is on the table, it is better to use "wipe off the table".
2014-01-08 22:47:57 補充：
The preposition is very difficult to explain in word. You have to use or see them in the context often, so you can get a feeling to use it right. Like "on" and "up" in Chinese, they all have the meaning of "上面". How do you know when to use which?