I have added lemon zest a few times, and it depends on how much flavor you want. Adding it at flameout will give hints of it in the finished beer, and in the secondary will really give a strong citrus flavor (depending on how much you add though). I just brewed an American Wheat and wanted some citrus background to it. I zested 3 lemons and 3 oranges, soaked them in vodka, and added it to the secondary. After 3 weeks in the bottle, it is way too much citrus flavor. Next time i would just add zest of 1-2 of each and add it to the boil.
Lois: "Oh, my God! You can only play the piano when you're drunk!"
Peter: "Now that's not true! I can also vomit, fall down and make dirty calls to your sister when I'm drunk!"
Primary#1- Pale Ale
Bottle conditioning- Citrus Wheat
Drinking-Mild, Kolsch, Sweet Stout
Upcoming- Red Rocket clone, robust porter