It's a game. If it's friends that you're gaming with, there is nothing about a game that is worth jeopardizing your friendship over. This goes for both the cheater and the suspecting observer.
That said, I game in a group (and have always gamed in groups) where we roll our own dice (sometimes not even sitting at the same table), and usually never where the DM can see it. And we don't care. It's a game. The point of the game is to have fun, not compete (some DM's my feel differently

).
It's pretty typical that rolls get fudged (not by me),
especially with ability scores. One DM I know is particularly fond of the roll-4-drop-1-do-it-all-over-again-if-you-don't-like-it technique. And everyone knows not to keep rolling until they have all 18's because that's not fun.
As for cheating to avoid dying, we also understand that the DM is trying to tell a story. If the whole party dies and the campaign ends, the DM doesn't get to tell his story anymore. This means we want the DM to roll secretly (to give him the "out" should the




hit the fan), and we're going to trust him. My character may die, but there's always a way to get back into the DM's story, because that's what he wants, too.
Why cheat? It's so preposterous that if anyone were to do it blatantly, we'd probably laugh at them for being retards. As a matter of fact, I am pretty positive that there is a bag of loaded d6's in the group. The owner denies it, but everyone in the group pretty much assumes each die roll 6's half the time. In cases where a character rolls five 1's in a dice-pool system (your master cooking attempt turns into a lethal inferno), the DM will frequently call for a reroll with the "special" dice (with loud exclamations of "they're not loaded! I swear!" from the owner). The table gets very nervous when the special bag doesn't show up for the session.