I don't drill holes, holes lead to rust, the trim rubbing flush against the paint leads to rust... and I hate rust.
Now a days, I back all my trim with 3M double sided tape. You have to take time to make sure the trim fits right and mark where you want it to go, as you only have one shot to get it right. I didn't take pics of the truck trim, but I installed it like I did on my mustang (starts at post 77):
http://67-72chevytrucks.com/vboard/s...=526257&page=4
I always leave the bottoms open or slotted, depending on length of trim or emblem, to let any water that may ever get back there to drop out.