Is there a certain point where you're "locked" into an ending? The game is fun, but not "replay six times for all the endings" fun. But if there's a certain point where I can go back to and replay only a fraction of the game, it's fun enough for that.
"I have an idea. It starts with 's' and ends with 'litting their throats.'" ~Belkar from The Order of the Stick
Towards the end of the game, you'll visit several areas with the Reason holders. Isamu's is the first at the Amala Temple. After beating the three bosses here, you'll have a small conversation inside the main temple with Isamu & Hijiri. It's here that the first ending can be lost, so I guess this is your 'point of no return'. However, it's then different for the other reasons. Chiaki's is Minafushiro, Hikawa's is the Diet Building, and for the TDE, you must beat the Labyrinth of Amala before entering the Tower of Kagutsuchi. However, for Neutral & Demon, you'll have to play through at least twice, as these two kind of contradict each other, and are determined by decisions throughout the whole game.
"You can not take to the skies with your feet on the ground."
Actually, I believe there is an event in Yoyogi Park where you can turn down the opportunity for the neutral ending, but I forget the details. Thus, if you want to get all six endings, you'll need to save before that conversation.
I think it's the one where you are asked if you are afraid and if you have the heart of a demon, or something like that.
Then play up to the Reason bosses with a separate fork for each of them. Make separate saves for each other ending, and then save once more before entering the final dungeon. I recommend playing "for real" for the True Demon Ending, since that one requires the most work. If you're too tired of working to regain all the nifty demons you built up to take on the secret final boss for TDE, you can either just enjoy the fact that the regular final boss is wimpy when you go for the other endings, or you can be cheap and Gameshark your way to those.
Once you enter the final dungeon, your ending is locked, by the way. I'm not sure if it counts if you enter the back door to get the treasure hidden in the basement, but definitely be sure to save before going in the front door if you want the option of the neutral ending (which is my favorite).