Yeah, what they said! I spent more than 20 years building machine tools and learning how to handle a level. The levels we were using were calibrated to arc seconds, 3,600 times more accurate than what you're using. But they also cost 100 to 1000 times more than your meter. As long as you always set up the same way, you're good to go. The business of turning the level end-for-end is known in the business as bucking the level. In my opinion that sort of nonsense should be reserved for machine tool builders and other anal retentive types. Like Jenny said, it's all relative.

What may also be of interest is that a .25 degree error is only .004" per inch of chord. So how bad is it really?

And finally, all this checking; left, right, reversed left, right is simply telling you more about artichokes than you need to know about squirrels. If you're comfortable that the wing is true, in and of itself, then cover the bird and let us know how she performs in the air. My guess at this point is that she'll be just fine.