We just had this
argument on RPGnet, actually. I'll report my thoughts:
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Taking 20 works less well for trapfinding than for searching for treasure, secret doors, and such, IMO. I've been playing through the Age of Worms adventure path, and some of the dungeons we've done have been many hundreds of 5x5 squares big. Taking 20 as you walk down the hall would take a lot of in-game time, but on the other hand rolling a Search check for each square would take a lot of out-of-game time. Presumably the RAW answer is to habitually take 10 while walking through the dungeon.
The problem is that taking 10 and taking 20 take the randomness out. That's the point of the rules, and sometimes it's really a good idea, but when finding traps, I can see how you wouldn't like it. I'm not fond of it myself. If taking 20 the rogue will find all traps he is numerically able to find, and if taking 10 he's going to miss them the vast majority of the time, unless they are really easy to find. (At 7th level my guy has about +13 or so to his check; I'd be surprised if most traps this far into the adventure path weren't DC 25+. By habitually taking 10, I'd never find any at all.)
That's bad for two reasons, IMO: firstly, randomness in trap detection feels like an improtant part of D&D. Secondly, it forces you to either roll for each and every square (awful) or take 20 (not always practicable. Say when invading an enemy stronghold; taking more time lets them respond better. Why there are deadly traps in frequently-used corridors isn't clear to me, but hey, genre tropes, right?) because taking 10 will get party members dead.
So in my group we're using a bit of a houserule--the rogue rolls once for a general area, against the DC of the only trap in the area. He doesn't have to point to the specific square where he's searching, or describe his search procedure in advance. Instead, the check represents his searching each square, while simply not bothering to roll all the checks that can't possibly find anything. Presumably he wouldn't get the check if he were running flat out through the dungeon instead of carefully checking.
Now, it *would* be metagaming if a poor roll on that check promtped a second roll, because of the house-rule setting it up. The PC is just visually inspecting the path for tripwires, pressure plates, and so on. He has no reason to suspect that the absence of a tripwire here is more meaningful that the absence of one 10 feet back.
Part of the reason the GM agreed to this houserule, I think, is that we had a PC die in the very first session when I failed to declare I was searching a particular 5' square in a tunnel, activating a trap which did ~6d6 damage to a first level elven mage (she went splat.) I had been declaring searches for everything that looked vaguely interesting, but quite because they were annoying everyone by taking up screen time. And anyway, I wasn't declaring a search for each step of every hallway. (It never occurred to me how attention-whore-like the rogue's job is before I played this PC. Everyone has to stop and focus on him all the time while he searches for and disarms traps.)
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Of course we can and occasionally do take 20 on something that's very likely to contain traps or treasure, but taking 20 on every step of every hallway doesn't sound like as much fun as rolling only occasionally, to me. It's tons better then rolling for every 5 feet, though.