A range of differing length good quality levels makes for easy setting out and ultimately a good quality job.
Always check the levels before use by taking a reading on a surface with the level in both directions. The bubble will return to the same place for the same surface. If the bubble shows different results for the same surface then the level should be replaced.
Never lean you levels against a wall when not in use. They will fall to the floor and the jolt will cause them to become inaccurate causing your tiling job to be out.