Machiavelli said:
For roleplay purposes, in almost every situation it makes next to NO sense to cherrypick classes, unless you need those levels of those classes to qualify for a prestige class your character is striving for.
I hadn't realized everyone role-played such dedicated, focused characters who encounter no twists and turns at all in the course of their lives. I feel almost ashamed that I did not follow up on my early interests, first in law, and then in computer science, before finally deciding to pursue economics in college.
The following is a little cliched and results in a truly sub-optimal character (I can't imagine any halfway competent powergaming munchkin
choosing to pursue this path), but here you go:
he/she decides one day to be a monk
The character was an orphan and raised by a religious order. He never expected to lead anything other than a life of quiet meditation and self-perfection, until the day a horde of savage grimlocks sacked his monastery and slew many of his bretheren. Vowing revenge, he decided to pursue a path that would grant him greater powers of mass destruction.
He leaves the monastery and joins a warmage college
A year of intense study of the offensive arcane art took the edge off his anger. That, and the calming influence of a fellow student that he found himself falling in love with. However, she was the daughter of a rich and powerful noble, and had many wealthy suitors. To gain an edge in the contest for her hand in marriage, he immersed himself in the intrigues of the nobility, ever alert for the chance to earn a fortune himself.
after a year of classes he headed out to the courts of nobility to entertain and beguile the rich
Unfortunately, his bitterest rival, fearing that he would soon win the heart of the noble's daughter, plotted his murder. Fortunately, he was saved by a cleric, but as he recovered from his wounds, he heard news that his beloved and his rival would soon be wed. Heartbroken, he decided to break all ties with his former life and join the clergy.
then next year join the temple of a god
His new life brought him back into contact with some monks from the monastery where he was raised, who revealed to him that his parents had disappeared while searching for an artifact in the barbarian lands. Determined to discover if his parents were still alive, he traveled there and befriended the local tribes, eventually even adopting their ways.
before thrusting him/herself into the wilds to be first a brutally savage barbarian
His months of searching finally paid off, and he finally found the hidden valley that his parents died to protect. In a moment that has nothing to do with game rules but the DM chose to dictate because it pushes the story forward, he communes with the dead spirits of his parents and takes it upon himself to continue protecting the valley.
then a protector of nature
Unfortunately, the valley is attacked and destroyed by a horde of savage grimlocks (don't you just hate recurring villians?), because he did such a terrible job of picking classes, so he is nowhere near as effective as a character of his level should be. Fleeing from the destruction, he meets a wise old woodsman who teaches him special techinques used to fight grimlocks and other monstrous humanoids.
with a final few lessons at the skilled hands of a random ranger met in the woods
You can justify almost anything if you take the effort.
My worry then isn't so much that a munchkin is power-gaming, but that a DM is oblivious to the insanity.
Some DMs enjoy this kind of thing because they like to explore interesting combinations of character classes and actually go out of the way to encourage it.
if you take 1 level of Cleric just to have the ability and use it for some power-gaming purpose (Divine feats, et al.), you should be promptly bitch-slapped back into playable fantasy by your DM. Clerics tend to be "clerical" for life, and should be inevitablity drawn to add more levels if not pressed onward to a higher calling (such as a PrC with Cleric prerequisites).
I think tastes vary too much for this to be a universal truth. Different people have different definitions of "playable fantasy", and not all DMs are against power-gaming (some even actively encourage it). Some do not see anything special about the cleric class that requires you to be one "for life". However, if your character is primarily focused on spellcasting, there are often good mechanical reasons to stay in a single class.