When I am running d20 (D&D, Modern, and Call of Cthulhu, to date), if the situation warrants the player character taking 10 and that will succeed, I usually don't even ask for a check. I simply say, "you all cross the narrow ledge without incident," or "you find the hastily-hidden papers taped to the underside of the desk."
In both cases, and despite the possibility of falling off the ledge, I'd let them take 20, if they want to spend the time. In the former case, I'd see that as crawling across, tying themselves together, and otherwise being exceptionally careful. In the latter case, they'd eventually find it anyway (that's a more classical "take 20" example).
Now, if there's sudden gusts of wind in the chasm or sniping duergar hiding above, taking 20's not an option - although I'll let them declare they're attempting it. They'll take their precautions, and then instead of a Balance check, it might be a Reflex save to react to the sudden-and-heretofore-unknown gust or a more difficult Balance check to deal with heavy crossbow bolts appearing in their sides.
If it's obvious that taking 20's not an option - the gusts are continual, for example - then of course I'll just say "you can't do that because of <x> or <y>."
Both mechanisms are, in general, just to save playing time, which has become more and more a precious commodity as the years went by. The only time it's not is the D&D rogue with Skill Mastery or the T20 service character with PMOS. That "take 10 even in stressful situations" can go a *LONG* way to make the character's actions automatic against a lower-level or cross-class opponent.
Take a L12 rogue versus a L12 fighter. The rogue has taken maximum ranks (15) in Hide and Move Silently, has an 18 Dex, and has lost his stealth-bonus cloak to the thessalhydra last session. He has +19 to his H/MS rolls, and so he could roll anywhere from a 20 to a 39.
The fighter (who has a really high Int) has taken maximum ranks (7.5) in Spot and Listen, has a 12 Wis, and has no bonus items, either. He has +8 to his S/L rolls, so he could roll anywhere from a 9 to a 28. His success chance to spot the hiding rogue is approximately 11%. (Actually, exactly 11.25%; I can provide anyone interested with the statistics for that if you want...)
Now, if the rogue has Skill Mastery for H/MS, he will always roll a 10 for a total of 29. The fighter *cannot* succeed in his observation attempt.
It can make a huge difference.