Some English usage and style commentators are unperturbed by the widespread use of as to as a catch-all preposition; others are aghast. The best approach would seem to lie somewhere between their respective reactions: to neither avoid it outright nor open the floodgates. The careful writer will learn to use it discerningly.
First, we can probably agree that as to is fine when introducing a new subject, or returning to a subject that was mentioned only briefly before:
As to the lab’s upcoming experiment, we’ll just have to wait and see.
As to the cost of living on the island, that’s something worth investigating.
Here it is used in much the same way one might use concerning, regarding, with regard to, on the subject of, on the matter of, on the question of, or as for (which to my ear is slightly less formal than as to). Note that a simple rearrangement can sometimes do away with as to altogether:
The cost of living on the island is [something] worth investigating.
The disagreements arise in its other common application, as a compound preposition. This usage has a long and impeccable history: the Merriam-Webster Dictionary of English Usage contains lines by Jane Austen, Henry James, Daniel Defoe, George Bernard Shaw and others, all using as to in a manner apt to horrify the fussier grammarians.
Unfortunately, the term lends itself to overuse, especially as a default preposition for writers who don’t know or have forgotten a more suitable preposition:
We are awaiting confirmation as to the meeting schedule.
We are awaiting confirmation of the meeting schedule.
It is essential to have detailed guidelines as to the project structure.
It is essential to have detailed guidelines on the project structure.
The problem with using as to in this way is that it obscures the relationship between the parts it’s supposed to connect. When it displaces a more direct preposition for no good reason, it can weaken the sentence. Its vagueness is mistaken for versatility, even vitality.
As to often pops up before words like what, which, how, who, whom, why, and whether:
It was anyone’s guess as to whether it would snow.
She wasn’t sure as to which cat was her favourite.
The pilots had no idea as to why the plane malfunctioned.
In The King’s English the Fowler brothers called this usage an “absurd prevailing abuse”, but it is probably not wrong in most readers’ eyes. Still, the above lines tend to read better, being plainer, without as to.
There is no unbreakable rule about using as to as a preposition, but there are good reasons for being alert to how and when you might be inclined to do so.