I always find watching the whole show night after night difficult. AS LONG AS YOU CAN DO THIS WITHOUT AFFECTING THE CALLING OF THE SHOW - and you should always watch the whole show, but you can focus you eye for giving notes by breaking down the show in different ways . . .
Ways to spice it up . . .
1) Focus on one character
2) Focus on the odd scenes, the even scenes
3) Focus on one actor (if they play multiple characters)
4) Watch the show focus on one element of design (lights, scenic, costumes, props, sounds)
5) Watch the crew focus on the run crew (or what they do on how it effects the onstage picture)
6) Watch the visual picture of the show (as if you turned down the volume)
7) Listen to the show (Watch as little as possible, but pay attention to what you are hearing)
Watch the show paying attention to tempo
9) Before a show, review notes the director gave during rehearsal, tech, previews - see how many of these notes can be regiven.
10) Be on book - see how close they are to the script during a long run.
Hope that helps.