President Trump's lawyers do not want President Trump to sit down with the Mueller investigators. They are wise to not want that. The President has a history of not quite being honest, even in depositions. Let's not forget he based his wealth on what he felt like.
But I think the President should keep his word. He has been very open about his willingness to sit with the Mueller team and, frankly, it will look like he has something to hide if he refuses. The President has the opportunity to put this matter behind him once and for all by providing a disciplined performance before the Mueller investigators. Failing to do so is just going to drag this out, raise questions about what he might be hiding, and provide Democrats additional ammunition with which to argue he collaborated with the Russians or should otherwise be impeached.
President Trump should keep his word. He said he would testify under oath. He should do just that.