It's said that after a certain amount of drop (more than 1" maybe?) that the stock struts start to get "overworked" with excessive spring oscillation.
I don't think your struts will die instantly, but they'll get more of a workout and see a reduced life...
Aloha,
Richard...
P.S. I'm running H&R Sport Springs (1.5" + drop after settling) with the stock struts. I'm assuming the stock struts will give up the ghost sooner or later, but I'll run them into the ground before I can afford new struts... BTW I'm not especially concerned with handling, just the aesthetic appeal of a lowered car. If you're going to go for handling the stock struts should go (From what I've been told by the track day gurus).