I think any strategies with item placements will depend mostly on the intended difficulty of the game.
Reborn tends to have loads of hidden items, but they're mostly the everyday potions/Poke Balls (which are harder to buy) The general goal of Reborn is to rely more on Pokemon's natural movesets, so when
Uranium's item placement didn't stand out to me much, but what did stand out was that TMs were often placed about as casually as an item in a route, and had pretty good ones, too. (there usually wasn't more than two per location, though) Uranium tended to involve high-leveled opponents, so giving people strong moves and encouraging strategy/making it easier to grind helped it out. High-demand items like the Lucky Egg and Exp. Share were given for taking part in something else in the game. (Exp Share was with the Garlikid Sidequest, Lucky Egg was given in the Day Care) It's a good way to make sure an item won't be missed, but I don't think that's something that needs saying-Exp Share has been a quest item in every gen, I think.
Ethereal Gates had a mix of the two-there were easy, classic items out there, but also TMs and type-boosting items, if you looked in the right places. (though I could never find them myself)
Edge Rising, being a faster game, put the high-demand items (Lucky Egg, evo stone, Exp Share, Leftover) relatively early on, but with a visible challenge for each one, usually a battle in front of it. (well, the Unlucky Egg didn't have a challenge, but it looked like it would be) There were less regular items about, but there was quite an abundance of money to buy them with,
But I think we can all agree that we need to have at least one item in a trash can at some point.