The value of site searching rainbow pretenders

[Tyrant]

If your pretender has only a few paths, radar spells are ok, but for a true rainbow manual searching is the way to go because you are searching for many different paths with one turn's effort. If you spent the points to get a rainbow mage, you made him for searching, and that's what he should do. Otherwise you should have taken a Great Sage.

Shovah is completely correct about 1-2 level sites being by far the most common, but on the other hand the many of the higher ones are very very good, so it is a tough call on whether to go 2 or 3. If i remember correctly, and if D3 hasn't changed it, and if the map does not have special sites, there are only a handful of 4s, so you it's pretty safe to go 3 and be confident that you aren't missing much.
The question is, how many paths can you afford to take 3 in, and that depends on the nation, the game settings, and the rest of your plan.

I reccommend a Master Lich as as rainbow pretender, especially for a non-death nation- start dominion 3, 20/path, immortal so you don't have to worry about his safety, and useful in other roles later in the game, or early if your plans go awry.

Others play it differently, but i use the radar spells only sparingly in the mid/late game to target "likely" spots- ie provinces that have screwy scales that hint at hidden sites, rough terrain that has no sites after search by my nationals, or conquered territory that i believe has been partially searched by my foe and may be dangerous to search manualy. Also, if i randomly get ahold of a single caster who can use a radar spell in a path that i cannot otherwise do, i will set him to radar searching. For instance, if i'm playing an astral nation without air, my first Harbinger will usually spend most of his life in a lab casting Auspex.

The magic/arms balance has swung heavily in favor of arms between DomII and D3, but gem income is still a decisive factor and finding more sites faster is a huge advantage IF you can survive long enough to do the research to put it into play. For this reason, i consider rainbows to be an excellent strategy, but riskier than the two main alternatives- SC pretender and high bless. If some smart alec with double 9 blessed sacreds or killer Naharraja jumps you on turn 8 you may be screwed, but if your regular forces are good enough that you can defend yourself you will gain an increasing edge as the game goes on.