I think it depends on the type of fish you are adding. If you are adding a hardy fish like a damsel or chromis then I don't think you need to worry about turning off the lights or anything like that.
However, if you are adding a really delicate fish that doesn't ship well, spooks easily and is generally very shy, such as firefish, dartfish, some wrasse (basically any fish that have a really bad reputation for jumping or panicing) then I would turn the lights off for the remainder of the day, leaving a light on outside the tank. The extra light will give the fish something to see with but the residential fish will be looking for their night spots as well and should generally ignore the newcomer. The next day I'll keep to the regular lighting and feeding schedual but will avoid putting my hands in the tank for a few days unless absolutely neccessary.