If I'm playing straight D&D, the ability score tables I use are (closest to, but a little more complex than) the ones from Swords & Wizardry:
3–8, −1
9–12, ±0
13–18, +1
If you're rolling 3d6 down the line, each stat has about a 25% chance of a −1 penalty, a 50% chance of no modifier, and a 25%...