No! Do NOT use site searching spells. You will waste HUNDREDS upon hundreds of gems and mage turns doing that. A level 1 mage will find almost every site in the game--astral is the only exception.
Actually, a level 1 will only find around 60% of sites, and that's pretty uniform across paths. A level 2 will find around 90%.
As to whether you're burning mage-turns or just gems, that mostly comes down to your national paths. If you don't have a lot of mages with multiple low paths, you'll be sending multiple mages out to search so the mage-turn argument breaks down. The gems argument is not as clearcut as it first seems either, as it takes at least twice as long to search for a single path with roving mages instead of spells, and those extra turns you'll not be gaining income from whatever sites you find. So while the spells are expensive, they're effectively less expensive than the list price because of their speed. They also don't require you putting your mage at risk to search, so for rare mages that's a major point in their favor. They also can be handy when an event suggests a hidden site is lurking in a particular area.
Generally, I'd not recommend going as crazy with site-searching spells in Dom4 as in Dom3, particularly for nations with mages boasting multiple low paths. But the sitesearching spells have their place, and their uses. And as Il Palazzo points out, if you don't use them, then how will you know when you've found everything?!?!?!?