Which do you do? I have been building for my players, taking their characters abilities and level I to account, and trying to avoid killing them. I am thinking of stopping that, and starting to build a natural world for them to face, whether it kills them or not.
Anyone who does the first, why?
'Build for players' or 'Tailored' encounters is certainly something I've done a lot. Starting with 3e CR, there were explicit guidelines to help you do so. They didn't work very well, but they were there. But, even back in 1e, I'd try to piece together from tables of monsters of a particular 'level,' from HD, exp values, special abilities, etc, etc, how challenging a monster would be and what would be 'right' for a 1st-level dungeon (or the 1st dungeon level: I had this idea as a kid that dungeons were literally supposed get deadlier and have better treasure the deeper you went). 4e, of course, made it a snap to toss together a level-appropriate encounter.
One reason to do it is the hammer & nail reason. You're given encounter building guidelines, so you build encounters to them. It's not a great reason or anything, but it's natural enough. A better reasons comes when you realize that the game is neither for your entertainment, nor an objective simulation you're 'judging,' but for the express purpose of the players having fun. That's a basis for all sorts of game-philosopher reasons that amount to the same thing. It can be a way of running a more enjoyable game.
Anyone who does the second, any tips for me?
"Build natural,' or "status quo?" Yes, of course. When you're designing a whole setting (with or without players lined up to play in it), for instance, there's no party to gauge it against, so it's a matter of what you want to be there for whatever other (creative, presumably) reasons. There'll still be tailoring (or bored players or dead PCs), but it's up to the players to tailor their approach (or rapid retreat) to what they encounter, instead of you to tailor encounters to them. It might seem a little unfair that in the tailored game, you have full access to the PC's capabilities, and are able to decide on the monsters', while in a status-quo game the PCs know only what you reveal to them about the potential challenges. It is. Status quo isn't supposed to be fair. But it's still supposed to be fun - you just trot it out for players who chafe at fairness.
So, really, either way you build for players, it's just some players hate to think that you tailored an encounter to be challenging instead of just statting out what was 'really there' (ignoring that nothing was or is there, you just made it up). Good on them for being so immersed. Tailor your campaign to them by telegraphing encounters that are well above and below their capabilities, so they can engage in 'smart play' and bully the weak while running from, tricking, or currying favor with the strong.
(Cynical? Me?)