Once upon a time there were a million towers infested with orcs, goblins, trolls evil mages or demons and in each tower was a princess that needed saving. In each tower was a chief, boss, elite, dark, grand, giant, master, or lord this or that with a pile of loot. A million adventuring parties went into these million towers and kicked all the orcs, goblins, trolls evil mages or demons asses and slew the chief, boss, elite, dark, grand, giant, master, or lord this and that. Woo hoo. It's time to count the coins and divvy up the magic items. But what magic items are there? How many? Oh yeah, don't forget the princess.
In the 1st edition Monster Manual each monster has a treasure type. Roll on the table and see what is generated. This is an interesting way to do it, but I don't find it an effective system for an ongoing campaign. I still use it to generate ideas. What I will do a lot of times is see what is populating my dungeon, roll on the treasure type table, then pick and choose what I like or alter something if I believe it is too powerful or too weak. Then instead of piling it up at the end or do the kill a monster, grab the treasure thing I try to make it more organic. Example, if I roll that a group of bugbear's in a cavern and they have a +2 sword and a necklace of missiles I would probably give the leader of the bugbears the sword to use and have the necklace on the corpse that fell into a pit trap. If nothing else to give the magic items a variety of discovery.
On pages 92 and 93 in the 1st ed. DMG a small, venomous rant against 'Monte Hall' style games and how the rules dictate there are no rewards for this kind of play. "No reasonable opponents, no rewards, nothing!" Then the next paragraph goes after killer dungeons, "Killer dungeons are a travesty of the role-playing adventure game, for there is no development and identification with carefully nurtured player persona." My favorite line afterwards is how a sadistic referee takes unholy delight in slaughtering hordes of hapless characters. Great line.
So we have the two extremes kinda sorta. To me the extremes would be Monte Haul and Ebenezer Scrooge, who will give you nothing. Again, I speak of this again and again about players expectations. If they expect to be fed a steady diet of magic items to tinker with, then it's important of the GM to provide a balanced way of doing this. Keep the power level of the items consistent with their level of play and risk. But if you have players willing to go for a more realistic campaign then there may be no magic items at all. Well crafted items become the 'magic items'. And if you look at some of the items listed in the DMG they are not magical at all they are just high quality items. Examples given on pg. 116 were boots and cloaks of elvenkind and +3 dwarven war hammer. Even the low level items, like any +1 weapon, could be argued to not be magical at all, but rather just made very well. But magic item creation is another topic for another blog.
I randomly selected two canned dungeons to compare. First there is T1 The Village of Hommlet (I had it out since Bat in the Attic recently did a blog on it) and then I chose a more recent dungeon Dragora's Dungeon. Both are 1st level adventures. In T1 the magic items are owned by experienced adventurers in the village, but all of them own some. Most own three of more items. So this sets precedence that magic items are common. This is a small village yet almost a dozen magic items can be found. And in the moathouse itself there are magical items. Also in Dragora's Dungeon there are a handful of magic items. So if a party of 1st level characters goes through a dungeon successfully then they will leave with a group of magic items in each instance. Two or three adventures and already the players will begin getting choosy in their magic item selection.
Depending on what system and style of play you prefer I think D&D is set up for the Monty Haul scenario. If you are providing a handful of magic items in a 1st level dungeon then the next one will have a few more or a bit more powerful items. Some games I've been in and GMed rely on magic as a form of luxury and technology. In a GURPS fantasy setting, magic items replace technology. The lesser, but useful magic items are affordable even to the lower class. In D&D the magic items are constructed to make a person more powerful.
Then there are games like Pendragon who shun magic items all together. You know that there will be no treasure hordes of magic items, but you may find something so powerful that if the player uses its power the consequences could be vast and devastating.
This is something I believe is vital to decide before a campaign begins, choosing the scarcity of magic items and their use in the community. Game on!