dtgreene: Abusing the bank for interest is just silly. I saw a speedrun for MM3 (which has a simalar bank and a gold-to-experience fountain) in which the player got a few characters to level 200, which is absurd.
I think the games have partially coded (but nonfunctional) time limits coded in that would prevent such bank abuse.
The problem is that there is no reasonable way to get enough money to get your experience.
Whenever I play World of Xeen, I set a few ground rules. One that I have considered is no leveling up past 50. (Level + events and fountains acceptable) Another is that I pretend the late game +50 to all stats less than 50 doesn't exist, as I don't like the effect it has on my gameplay.
destructa: I'm no speedrunner...a thorough play will take me a few days or a week now. Level 200? Wow. I have always made use of the bank; most of the time you just don't need all that gold & gems on you, so might as well let it accumulate. But only while there is still adventuring to be done; no fun just waiting around for payday. Who could turn down that kind of interest in real life?
My ground rule was to save any permanent level bonuses until I had completed almost everything and trained up to the limit of the party's gold or xp. I forget which was the limiting factor. I was tempted to just take them; +5 levels changes the balance a lot more at Level 10 than at Level 60 or 70. Revisiting those locations (like Eastern Tower) collecting them was a very late task, probably just before the Megadragon and going to endgame.
Now, aging the party 10 years doing labor for the guy in Castleview is just silly!
For the Isles of Terra speedrun, I think it was more on the order of 100 years. Thing is, you get 200 million gold per character (to convert to experience) plus enough money to actually train to level 200 (26467000 per chraracter). Just watch out for integer overflow.
In World of Xeen, you can't convert gold to experience, so you can't just use this trick to get level 200 characters early; you still have to earn all that experience.
What I like to do is complete Dragon Tower as soon as possible, this way I can get the 20 million experience for everyone when it still matters. I can then get access to the 50 (!) level fountain (using Teleport because the enemies in the Dragon Clouds aren't manageable at low levels) and reach natural levels in the 40s early.
One reason for stopping at level 50 is that you actually have to play through the darkside to get access to any place that would let you train higher. (Note that, at level 50 with a +50 bonus, the Labyrinth of Lost Souls is no longer a serious challenge.)