Well, it depends alot on the particular engine. In general, as long as the oil stays below the crank throws in the pan, there is no downside to overfilling. If the crank-throws get into the oil, it is whipped into foam, with a vareity of bad consequences (lack of lubrication, seal damage from crankcase pressure, etc.)
Having said that, a lot of engines can be filled over the limit somewhat(most GM)...and a handful are considered to do better slightly underfilled (Northstar's). What engine do you have? You might check
I like keeping the oil topped up, mainly for the additive boosting value...and for a little insurance in case I have to take a long trip or something similar. The downside is keeping partially full oil bottles around...I keep a full spare in the car typically and keep the partial top-off bottle at home.