I used to be quite hostile to the 'Elder Scrolls IV' approach, but actually running 4e I've learnt that in that system it seems best to treat the game as a story and scale threat to characters, within reason, the way you say. In other games/D&D variants (AD&D, PF, LL) I don't really do that, or only minimally, in the 'there are newbie-survivable areas, and tougher areas' sense. Even in 4e I have established areas that are 'above the pay grade' of the PCs, but they have no great reason to go there and get killed, as you say - or if they do have reason, at least they can find out it's a deadly area and I'd often give them a chance: assaulting the Zhentarim fortress of Darkhold at 9th level would be unwise, but you migh be able to sneak in, or negotiate.
As you note, there's always "within reason". If the party sets their goal to whack a high level guy, I ain't going to lower his level to meet them. But assuming the party sets level-appropriate goals, they'll get level appropriate challenges.
My ideas on my practice come from what I sense as self-selecting level appropriateness in sandbox play. A 1st level party of reasonable sense, will find stuff they can beat to adventure on. Only really stupid players attack the archmage on their first day out. So if the players are trying to select content that is level appropriate, why not smooth that out a bit for them and cut to the chase.