My decision on which stat to "dump" is affected by the class of the character, the concept I had when designing them, and just how low the dump stat is.
STRENGTH
If my lowest stat is a 10 or an 11 (quite possible with 4d6 or with stat buy), and I am not creating a melee type, then I'll often "dump" here. I never put a stat with a penalty into Strength, however.
CONSTITUTION
This is never a dump stat. HPs are too important
DEXTERITY
When creating Fighters, Clerics and Paladins, Dexterity tends to be a dump stat due to the heavy armour those characters will ultimately wear.
INTELLIGENCE
Much like Strength, I'll put a 10 or an 11 here, but nothing lower: while skills are not a major focus of 3E, I find that it's still not a good idea to have a negative here.
WISDOM
A good choice for Rogues and Bards, as it often fits quite well with the concept behind such characters. Also surprisingly good as an option for Sorcerers and Wizards, since they have high WILL saves even with a Wisdom penalty. It also makes for good role-playing opportunities for those classes.
CHARISMA
If my lowest stat is a 10 or 11 then I have few qualms putting it into Charisma (unless, of course, it is an important class ability). I rarely create characters with a Charisma lower than that, unless playing a Dwarf or Half-Orc. However, Rangers and Monks tend to need this stat less than any other, so it's a good choice for them.