It's really simple -- if you're not getting enough out of high secondary stats to justify lowering your primary stat, you're better off bumping the primary (this is frequently the case). If you're getting -depressingly- little out of your secondary stats, you're usually better with the primary at 19 or 20.
In particular, characters who wear light armor and use dex/int as primary should frequently have a 20 int/dex. You're getting damage, AC, and to-hit out of your primary, so you'd better be getting a lot if you start with lower (that said, my genasi wizard went 17 str/19 Int, so I could start with a bump in wisdom and a respectable Str).
Characters that can get to-hit/damage from a secondary stat and otherwise get big bonuses from same should go for more balanced stats. Dwarven fighters are classic here -- and they're classic for a reason -- with the right feats, a dwarven fighter with 16/16/16 will hit more and do more damage on combat challenge and opportunity attacks (though not on normal attacks) than a fighter with a 20 in her main stat, due to a combination of +wis to hit and +2+con to damage, -and- get all the good dwarven stuff and very good defenses. And 18/18 dwarves can work well as well.
Taclords are another common upside down class, with a fair number of people doing a 18 or even a 20 Int and a lower strength. I'm not entirely down with this -- I think 18/18 is probably right (for, say, a Genasi taclord), but given the short length of most D&D fights given optimiszed characters, it's hard to justify lowering your ability to hit and damage with most of your encounters and dailies just to power up your at-wills (and -occasional- encounters). If and when you can build a taclord who only makes Str-based attacks on dailies, it might be worth finding a way to boost 2-3 attacks per day and going 16/20, though, saving the big boosts for daily attacks off Str, though.
So in sum, figure out what the minimum you need in secondary stats to be effective with those stats, then dump the rest into primary unless you've got a -very- good reason not to. With different class/race combinations having different ideas of what you need in secondary for being "effective".
In particular, characters who wear light armor and use dex/int as primary should frequently have a 20 int/dex. You're getting damage, AC, and to-hit out of your primary, so you'd better be getting a lot if you start with lower (that said, my genasi wizard went 17 str/19 Int, so I could start with a bump in wisdom and a respectable Str).
Characters that can get to-hit/damage from a secondary stat and otherwise get big bonuses from same should go for more balanced stats. Dwarven fighters are classic here -- and they're classic for a reason -- with the right feats, a dwarven fighter with 16/16/16 will hit more and do more damage on combat challenge and opportunity attacks (though not on normal attacks) than a fighter with a 20 in her main stat, due to a combination of +wis to hit and +2+con to damage, -and- get all the good dwarven stuff and very good defenses. And 18/18 dwarves can work well as well.
Taclords are another common upside down class, with a fair number of people doing a 18 or even a 20 Int and a lower strength. I'm not entirely down with this -- I think 18/18 is probably right (for, say, a Genasi taclord), but given the short length of most D&D fights given optimiszed characters, it's hard to justify lowering your ability to hit and damage with most of your encounters and dailies just to power up your at-wills (and -occasional- encounters). If and when you can build a taclord who only makes Str-based attacks on dailies, it might be worth finding a way to boost 2-3 attacks per day and going 16/20, though, saving the big boosts for daily attacks off Str, though.
So in sum, figure out what the minimum you need in secondary stats to be effective with those stats, then dump the rest into primary unless you've got a -very- good reason not to. With different class/race combinations having different ideas of what you need in secondary for being "effective".