Usually this works well, though sometimes I have to start over with a tougher party. However, I often have the experience that my party can take all the monsters in some town with ease, but can't get to them because the party can't open the doors. Frankly, I don't see the fun of casting half a dozen Unlocks just to get through the next door, even for players who started parties matching the scenario. So my personal taste is to set all towns to low level, regardless of the scenario level, so all the doors open easily. Doors you want to keep shut for plot reasons you control with specials anyway, so this decision will not impact the plot.
Traps are a quite different story.
A trap is passive, because the party isn't forced to take it. Traps of the first sort can be used as puzzles. For example, a dragon hoard with a high multi-bang trap doing 500HP damage or so is a real puzzle for the party. Short of that, traps in quiescent situations can add verisimilitude, but don't add any challenge to the play. So set the level to what the plot would have the trap-setter do.