there are two ways a sprocket can be "out"...on it's axis (forward and backward) and laterally (side to side).
I've noticed more than a few sprockets are actually warped...meaning: if you took it off and layed it on a perfectly flat surface, you'll see a high side where the teeth arent touching the flat surface, but they do touch on the opposite side.
(these cheap-o things are not machined...they're STAMPED out.)
i use a coarse file, and file the sharp edges off the sides of the teeth. then i do just like Bob says, and make sure that my chain tensioner is where it needs to be.

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