These days I'm used to checking every single precondition for every function since I got the habit from an OS programming course back at uni.

On the other hand, at the software engineering course we were taught that a common precondition should only be checked once, so for example, if a function is delegating to another function, the first function should check them but checking them again in the second one is redundant.

I do see the redundancy point, but I certainly feel it's safer to always check them, plus you don't have to keep track of where they were checked previously.

What's the best practice here?