Adding too much hops will cause the beer to taste grassy or oily. This can happen, but it normally happens when you dry hop for too long of a period and is not dependent on how much hops you use. Most brewers dry hop for less than two weeks so this is not normally an issue.
It's commonly recommended that dry hops be added to beer at fermentation temperature, somewhere around 68°F/20°C, which is widely believed to lead to the extraction of more hop oils.