[Unwise]

As with everything else in a well-balanced game, it's a trade-off.

If you've got a fire-2 mage (with no other paths) then it is almost certainly worth it to use Augury: he will only search one site every two turns (at best) if you do it manually, and he will never find the best fire sites. As Agrajag noted, as soon as you've found two sites with Augery (for example), then the operation is self-sustaining.

However, if you have a high-powered mage (say F2A1N3), then it is probably a waste to have them cast Augery because you have a decent chance of finding more sites manually than only fire sites through the spell.