mrmum, I think Cortina was meaning it works provided that you actually read what your thumb is pointing at.
I do however see advantages of doing it from memory. Especially in tricky airspace with lots of traffic to look out for. The more time your eyes spend looking for potential hazzards outside the better. Looking down on a checklist does not help.
I know you could probably turn that the other way round though and say its not as safe in case you miss something important. I agree with that too.