I usually try to maintain a water level that's even with my grain level during sparge. I've never had a stuck mash (even when using flaked oats). As mentioned, don't let it sink far below that though... You don't want to risk a very bad brew day.
Our beverage that art in kegs,
Hallowed be thy tap,
With thee supplied, we will imbibe,
At home as we do in the public house.
Give us this day our liquid bread
And forgive us our spills
As we forgive those who spill upon us.
Lead us not unto hangovers
But deliver us from overindulgence.
Buy a man a beer and he wastes an hour. Teach a man to brew and he wastes a lifetime.