I attached mine last weekend, and tbh I found it is better to apply the spoiler when the grill's on the car as there is a slight curve in my grill.
I had to bend the spoiler slightly to fit it properly, but this meant that I was able to drill the holes exactly where they needed to be, rather than putting stress on them when bending the grill.
I'm sure that both ways are good, but I'm pleased with my result
