KidSnide
Adventurer
I'd be OK with guidelines for high, average and low/no magic rewards.
The more I think about it, the more I think there are two different parts of guidelines.
Encounter guidelines need to tell the GM whether a given encounter will be trivial, definitely-winnable-but-resource-exhausting, challenging, near-impossible or impossible. That's mostly about figuring out the "level" of the encounter, with modifications if certain monsters are used. (E.g. monsters that can only be hit by silver or magic weapons will fight above their level if characters don't have the necessary equipment.)
In the same way, magic item guidelines need to tell the GM how the amount of magic in the hands of the PCs effect the PC's ability to handle encounters. In other words, how much magic do I need to give the PCs to let them fight as if they were a level higher.
Those are evaluation guidelines and I would like to see them as vigorous and accurate as 4e encounter guidelines. Completely separate from that is the question of how GMs select encounters and treasure.
One GM is going to select "level appropriate" encounters, while another GM is going to spread encounters of all levels around the game world and let the PCs figure out which ones they will take on. A third GM might create encounter tables for the various areas. These areas might have an effective "level" representing the challenge of the most common opponents, but that wouldn't preclude the possibility of encounters far outside that band.
In the same vein, some GMs will want predictable treasure, keeping the PCs at a certain wealth level (whether that level is high, medium, low or impoverished). Other GMs will want treasure that can be predicted (at least to a certain extent) based on the monsters faced so players can make an informed choice about whether the rewards for a given encounter are worth the risk. Random tables may be the choice of others.
In all of these cases, the GMs should have the tools to evaluate how difficult encounters are and how much treasure will affect the PC's capabilities. But using those tools to select monsters and treasure is just one of several perfectly viable design decisions.
-KS