Rolling ability scores in DnD can be a painfully tedious endeavor. If you don't watch every player roll his/her ability scores, you're likely to face at least one person in the group who has somehow miraculously rolled three 18's, two 17's, and a 16. This is especially true of younger players or rookie players. I try to stress one important fact to my players before we ever begin an adventure; your skill as player, not your ability scores, will determine your success in this campaign. Ability scores are just numbers on a piece of paper. So, why do some players get so hung up on having high stats? I think part of it lies in the fact that, until they've played the character in an adventure, the stats are the only method by which they can visually measure up their character. The stats, in essence, replace the imagination when envisioning the character in their mind.
There are many methods outlined in the Player's Handbook on generating ability scores and I won't go over them. Some purist DM's will say, "the only method you should use is rolling 3d6 six times and writing the numbers down." That's all well and good, but as a DM, I like to give my players at least some choice in who and what their characters will be. I use a simple point method in character creation. Each player has 72 "points" to divide as they see fit among the six ability scores. The results are usually pretty good. Sure, someone can have an 18, but when you have to split up those other 5 abilities with only 54 points, it becomes more of a challenge. I find that this method allows the players to have the versatility to be able to play the class they want, say a Ranger, while still having some semblance of balance in relation to their ability scores.
The key factor, in my opinion, is getting the players to realize that ability scores do not make the character who he/she is or will become. The play of that character will eventually determine his or her destiny. Ability scores, like everything else in the game, are merely tools to be utilized when necessary.
So, what are your thoughts on rolling ability scores? Do you as a DM give the player any liberty in determining their characters' stats or do you believe in letting the roll of the dice determine such factors?