I've got a 2004 V70 and purchased aftermarket roof rails (Euro style) (Perrycraft Aventura) These require drilling and screwing into the roof of the car. I'm wondering whether to drill the holes into the "channel" or not. Does anyone know?

 

Yes, definitely if you don’t have drill in your channel; you have to go for that.
Assemble cross bars and side rails using screw driver
Place the roof rack unit carefully on top of your car and adjust its position in center of the car.
Mark the point at the end of the side rail and then drilled the hole into that point through the roof of your car.
Insert a screw into the end of the roof rail and tighten it with a screwdriver.