I have no hard data to support it, but I am under the impression that many referees of D&D (in its various editions and variants) employ the rule that player characters, upon earning the experience points required to gain a level, must pay to undergo some professional training in order for the new level to take effect. As a referee I have never used this rule (probably because it does not exist in the Moldvay Basic rules, which was my first exposure to D&D rules), and as a player I have never had a referee use it in a campaign. Personally, I've never seen the point of it. It unnecessarily and inconveniently interrupts the flow of play, it doesn't accurately reflect reality or fiction (except perhaps martial arts movies), and it seems like a petty tactic to deprive player characters of their hard won treasure. I'm not a Monty Haul Dungeon Master, so I don't usually face the problem of player characters with more riches than they know what to do with. Treasure is sought because it is desirable. It is desirable because it enables adventurers to do more of what they like: carousing, buying supplies, flaunting their wealth, building strongholds, etc. things adventurers find enjoyable. It is also desirable because it enables adventurers to take care of necessary expenses: food, lodging, taxes, tithes, dues, healing potions, etc. I do not need metagame rules to create excuses for taking their treasure from them. The adventure they just survived and the experience points they earned is sufficient justification for gaining a level. They don't need a leveling up tax on top of that. Referees, if your players survived the hell you put them through and managed to get away with treasure, don't they deserve to enjoy it?
That's my view. What's yours?