Steeping tends to allow flavors to develop. I flavor lightly, generally 10-15% total (even less with flavorings from FlavourArt and Inawera), because I have quite a few in rotation and they'll be many weeks old before I finish them. They get more potent as they go along. It's cheaper that way too.
A complex mix will evolve a lot over time, as the different flavorings may hit their peak at different times. Mixes with creams, custards, and tobaccos seem to benefit from a longer wait. Periodic shaking and aerating can help.