The way I have seen it worked at many tables is thus:
Direct spells require line of sight, thus as long as the caster can see the target, he spell effect happens.
Indirect spells (like fireball, throwthower): require line of sight to the target*, if he can see the spot, he can cast.
Basically, because most indirect spell effects create "something" that does the damage, they can be "dodged" as appropriate. And spells such as Fireball can hit multiple targets if placed well (such as tossing a fireball into the center of a small room).
Direct spells, require you to actually see the target to establish a link, and transmit the mana directly to the target (Stunning, wounding, taking over their mind, etc). This means a foe hidng around a corner of a wall wouldn't be affected by a AOE direct spell...
Now, I can't point you to a direct rule, but that is the way I have seen it played out.