This is what I love about rolling with fixed stats (i.e. you don't choose where the 18 goes).
We have a method of rolling in most of my games which is adapted from late 1E where you choose your class before you roll and it uses different numbers of dice for different abilities, but your scores are fixed. You roll strength and that is your strength score, you can't pick another roll to put in strength because you are a Barbarian and it is higher.
In this scheme you choose your class before your roll and you are almost always going to have a 15+ in your main stat. However you may get an 18 in charisma as a wizard, while you have a 16 intelligence.
This creates some of the best role playing and it is true to life to a degree, what you want to do in terms of profession is not always what you are best at.