I've always considered WBL to be more of a suggestion than any sort of hard and fast rule.
I tend to dislike mechanically enforcing the character's relationship with the world, whether social or economic. It didn't work for me in GURPS, and it doesn't work for me in D20. I prefer that the character's relationship to the world be defined and limited by role-play. A character shouldn't be penalized for creatively finding wealth. The DM's job isn't to push the character through a series of staticly defined hurdles. The DM only need insure that resources are available for overcoming the sorts of problems that the players are expected to face. The the parties wealth lags or leads the WBL guidelines, I don't see this as a problem. It merely means that they either must slow down their progress toward certain goals until the accumulate the needed resources, or else if they have more than sufficient resources that they can adopt new and more challenging goals (and foes).
As long as you know that if your 10th level party is scrounging for +1 magic weapons that they probably will have more difficulty with a CR 10 monster (and especially certain ones) than normal, then you are good. Or conversely, if your 10th level party is walking around with the hand and eye of Vecna or your first level halfling rogue is carrying Sauron's master ring, then they might have more resources than is normally suggested by their level and they can be challenged appropriately. That's just a matter of taste, and either can be done well if the DM is thoughtful.
Some of the challenges in greater or lesser wealth by level might prove unexpected though. Just a few things to keep in mind from my experience.
You might be tempted to think that 'low magic' highly favors spellcasters, but thats true only of moderate levels of 'low magic'. Very low magic means that spellcasters don't get the AC boosting and CON boosting items that they often depend on, resulting in them being very fragile and very very fragile when unbuffed. For this reason, expect magic item crafting feats to be in high demand (and to potentially ruin your intention to run 'low magic').
On the opposite end of the spectrum, very very high wealth tends to turn a creative player from being an 'Indiana Jones' to to being a 'Rene Belloc' type. Why go down into the trap filled tomb when you can hire laborers to dig the whole thing up? Why fight the goblins in their lair, when you can hire an army 10 times as large as their whole tribe and lay seige to them? And so forth. Depending on your personality, you might find this more fun than running a dungeon focused campaign, but be warned that it can become very complicated to manage very quickly.
