In *SP* I use *auto*-spell-search for 2 reasons:
1. It's less micro, and I figure it might be more even with how the AI opponents search.
2. If you're a bit OCD like me, you'll end up haunted by the possibility that your level < 4 mage search *might* have missed something, and then end up going remote-casting over it to make sure
The time I would think casting is definitely best is when you've only got one level-2 certain-path mage available --- it seems to me a single(-unique)-path mage searching takes 2 turns per province and 2 turns of upkeep.