[militarist]

Sitesearching is definitely one of things you should plan during pretender design. My favorite strategy is taking sleeping rainbow pretender. I check what paths my mages have, and if I can produce mages with lvl 3 magic (or, especially two different level 3 paths on a mage) - I exclude these paths from rainbow design (for example MA Ermor rainbow in my latest game was designed with no paths in S and D), unless, of course, I need these paths for minor bless. So I usually have cheapest rainbow pretenders with many paths 3-4 lvl, which are useful for bless nations and just optimal for a nation which donesn't bid on their sacred units. And forging boots of flying for him is a very good idea.

It also depends on game settings. If research is difficult - I strongly prefer sleeping pretender taking better bless or scales ..If research is easy, then awake rainbow, which can give you both - research and sitesearching - can open many winning strategies for you (like relying your expansion and early wars on spellcasters support,not just raw power or bless units.)

As for manual sitesearching - you can easily calculate and compare...you need 2+ turns of a mage for manual sitesearching. And you need only 1 turn for spell. Each turn you spend you loose potential research. One more factor - it`s better to find sites earlier, as you will start getting gems earlier= will have more gems. All doms are about mathematics.
If you are betting on early finding many sites, it's also good to have a way to spend them, so some priority can be given to constr, or conj. research to optimize the strategy.