Since I installed an on/off switch to my spoiler motor, I usually drive with the spoiler in the down position most all of the time. I usually cruise on the high way between 70 and 80mph, depending on how the traffic is flowing and I have never experienced any ill handling effects what so ever.
If you do take it above 90mph an audible "alarm" will sound when the spoiler is in its down position.
In the extremely rare occasion when I'm caught in a rain shower, I manually switch the spoiler on so it will go up.
But as MMZ_Timelord pointed out, they didn't go to all that trouble for nothing. I just think the German engineers had "Autobahn" speeds more in mind, and that's why they don't warn you until you reach 90mph.