Depending on the type of game you and your players enjoy, fully defining BBEGs (or any encounters) too far in advance may cause you headaches later. If you are going to follow the Pathfinder AP approach, which is rather rigid in terms of how the adventure progresses (you must be level X by chapter Y), fully defining everything in advance is just fine.
On the other hand, if you prefer a more free form style game, allowing your players to skip areas or where a side trek can turn into a 3 level ordeal, you could have a problem. Meaning your level X boss may not be appropriate when your players finally get there.
To resolve this, you could just define the bosses as concepts. Iron Eye Lord Throvin becomes a male Chelixian human ranger (infiltrator) who prefers ranged combat with a bow and has some kind of fiendish creature as a bodyguard/companion. He may be anything from level 1 to level 20 with 10 mythic ranks depending on when the PCs get to him. You can pick out key feats and character options that help define his concept. Naturally, the closer your players get, the easier it is to figure out what level to make him. The session that they kick in the front door of Throvin's castle/hovel/dungeon, you'll know exactly what level to make him. And you'll also know what goodies he will need to have to avoid being "rocket tagged" by a level 15+ party.
Another consideration when building encounters: All of the rich complexity and interesting aspects of character classes is often entirely lost when seen by players through combat. All they see of monsters is AC, attack, damage, saves, etc. How those values get calculated is entirely hidden from players (unless you are showing them the stat blocks). So a fighter, a barbarian, a magus, and a melee inquisitor could end up looking exactly the same to the players - they hit often and hit hard. Yet the fighter is the simplest to play and the inquisitor is the most complex. For a GM, complex for no purpose is generally not good because you already have too much on your plate (you play the entire multiverse except the PCs). Consider what, if anything, your players will be able to see in exchange for the complexity of the class.