I agree. If filler is under the stickers on the back, or even a minimal amount is visibly apparent on the back, it shouldn't really matter, as long as the bat is graded to reflect that.
Where it would be cause for concern is if the filler was either highly visible on the face, and/or in the hitting area, or where the bat flexes around the handle and splice. With the latter two, there would be a genuine concern over possibly impaired performance.
Having said this, I'd expect a Pro or Grade 1 to have no filler whatsoever - even under the stickers.