I'd vote for bottling time...that's when I always add all my flavorings, and the instructions on most of them agree. I'd start with about half of what the directions say, though, and add more to taste if necessary...in very small increments. My worst experience was with spruce extract. The bottle said to add 2oz (the whole bottle) per 5 gallons of beer. That seemed a little much, so I looked at the recipe---Papazian's Goat Scrotum Porter---and he said 1oz. So to be on the safe side, I just went with 1oz. Now, I don't know if this brand of spruce essence was particularly strong, but to this day, nearly a year later, it tastes like pine tree in a bottle. I shudder to think what it would have tasted like if I'd followed the directions on the bottle...
So, yeah, be VERY careful! You can always add more, but you can't take it out once it's in there...