Keep in mind that if you use a bubble and it is on a tripod,you must level it while pointing in one direction.Then rotate it 90 deg,and level it again.These diferences must be split. Like using a transit.Anyway a level would only be used if you are shotting like a 9 shot panorama in my opinion.

"The happiness of a man in this life does not consist in the absence but in the mastery of his passions."