Totally depends on the game, but we like to roll dice. Because dice are harsh mistresses, we have various stipulations to avoid "bad" characters. In my first campaign, we used standard rolls, but you could default to the array after you rolled (making it the minimum). Two other DMs have methods they use to figure out the average value of the group, then give a bonus to everyone below that average. My current game has multiple characters per player, so players roll 3 sets, using 2 of them to build characters, preventing one set of bad rolls from ruining a character.