White Box D&D had, at most, + / - 1 for high or low ability scores, but later supplements changed that. By the time we got to AD&D, the + / 1 range was 4, and don't even mention Exceptional Strength for fighters and their sub-classes.
I tend to favor a range of + / - 3 for the games I run these days, but taking a step back from DMing (not completely, but in large part) for the summer-plus, has gotten me to look at ability score bonuses again.
When a 13-18 is a "plus one", actual scores become much less important. Depending on how heroic of a game you want to run, that plus one might be all you need. Conversely, it keeps the characters from being hosed by a horribly low score, as really, a minus one isn't that huge of a penalty.
How much of a role does ability score bonuses play in the campaigns you run?