You best "laser level' and easiest to operate are your eyes. Leveling
pictures with the proper hangers can be done any time, positioning is
the key, use your eyes.

Eyes are great for positioning and designing the layout. Leveling plays
tricks on your eyes - or in my case - eye. If you want them to look nice
to you - use you eyes - if you want a really straight setup - use a
level or laser level. Is a door or ceiling is crooked - it will cause
you to lay out other things crooked. Sounds like some folks on here are
a bit reactionary :-)