Hmmm, I'd say it depends in part on how your ride. If you've bent rails before, then it's probably not worth the risk of weakening the rail. On the other hand it might not make enough difference to matter for you, even on a lighter rail. I've drilled new holes so I could run the front idler wheels and scratchers, and it didn't cause any problems, but that was on my old sled with heavier rails than an AXYS. I wouldn't say never, just that you're taking some risk: obviously, if you bend a rail, that's going to cost you more than a new pair of scratchers. If you do drill, try to keep it in a "meatier" area as mentioned, and I'd add to keep it away from the edges – I can't think of a better way to weaken a rail than a hole close to the edge of a highly stressed area.