Serve have to cross?

I noticed, in some ping pong videos, that some people don't cross the ball when serving and recently someone told me that you must cross the ball when serving. So my question is, when serving, do you have to cross the ball? 

Hi Jack,

In doubles you have to serve from your right boxz to your opponents right box, or across the table.

In singles you can serve to anywhere from anywhere on the table as long as you are behind the end line of the table. 

The line down the middle of the table is only used for the doubles serve.