Really all you're trying to do is keep the game from being too easy or too hard throughout the entirety. The difficulty could maybe get slightly harder as you go, but typically this doesn't happen and the developer just tries to keep it constant. The reason you keep playing an RPG is not for increased difficulty, it's so that you can a) Accomplish tasks that used to be difficult easily at a later point, and b) you can see more of the storyline.
I read a book called Swords & Circuitry, which is about computer RPG design. It said that it's better to make the difficulty relative the player's level go up and down slightly as the game progresses. It's supposed to keep the player's interest better.
You can have a section of gameplay that's relatively easy because you're only fighting regular monsters and then follow it with a group of powerful boss monsters. And then you can have a simple errand quest. And then a cave with monsters that are all a little tough compared to the player's level, so that the player has to go back to an old area and do some grinding. And so on.