Many stylists avoid this phrasal preposition on grounds that it displaces too many ordinary, down-to-earth, single words: about <a rule about something>, at <surprised at how quickly>, by <surprised by the number>, for <same rules for individual taxpayers>, into <inquired into the problem>, of <unsure of the rule>, on <agree on a deadline>, to <the answer to the question>, and with <same is true with health insurance>. Inattentive writers—especially lawyers—will use as to in every one of those phrases, as opposed to the simpler prepositions. As to can be addictive.
Sometimes as to is completely superfluous: stylists know to use the phrase question whether, not question as to whether (or, almost equally bad, question of whether).
Because as to doesn’t immediately and specifically establish conceptual relationships, it can hamper comprehension. You’d be well advised to reserve as to for only one circumstance: starting a sentence that would otherwise start with the phrase With regard to. As to that particular usage—and only that one—the wording is entirely defensible.
The Chicago Manual of Style 313 (17th ed. 2017).
The Chicago Guide to Grammar, Usage, and Punctuation 240 (2016).
The Elements of Legal Style 104–05 (2d ed. 2002).
Garner’s Dictionary of Legal Usage 89 (3d ed. 2011).