This is one of those things that many just plain over think. From what I've seen, 55 and 62gr projectiles are the most commonly used. With all of the buzz lately about the performance of the 69 and 77gr projectiles, many are getting it in their heads that they want to be able to shoot these heavy weights and that's fine. What I don't see a whole lot of is guys shooting projectiles lighter than 45gr in their ARs; so the bottom line is that if you want to most practical flexibility, go with a 1:7 or 1:8 twist. Either will do fine and will cover most bases. As a general rule of thumb, a 1:9 twist will allow you to shoot up to 69gr projectiles. 1:7 and 1:8 will allow you to shoot the 77s as well.
Just keep in mind that a 1:9 twist isn't slow. It's just slower than 1:7 or 1:8