(For reference) I'm a candidate master.
However, I've been trying to improve the tactical side of my game, especially calculations and time-management.
One thing I do a lot is double-checking my calculations. While it obviously contributes to my time trouble, this practice has saved me from quite a few blunders.
I've been reading a book by GM Smirnov, Champion Psychology. He strongly insists that it's a very bad habit, as it is a waste of time and proves my lack of self-confidence.
I'm shocked that in my many years of training I've never came across such a blunt opinion (except in Kotov's books, which are, however, strongly criticized e.g. by Nunn in Secrets of Practical Chess).
So what's the deal here? Do strong players (2400+) really not double-check their calculations even in classical time-control?