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Anyone who saw Bird's demonstration of the barlowed laser at Snake Valley would have seen that focuser slop and misalignment makes no difference. That's the beauty of the technique. I guess if the screen is far enough away from the focal plane then you might see it make a difference, but in practice it's not likely to be a problem.
If in doubt, try and see: Loosen the retaining screw/ring holding the laser + barlow in the focuser and wiggle the barlowed laser around. You should see the light on the backreflection screen shift about but the shadow of the primary centre spot should stay put.
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