Depends on how much understanding you're talking about. I don't need to know how to build an x86 chip, but as an experienced and educated professional programm

Depends on how much understanding you're talking about. I don't need to know how to build an x86 chip, but as an experienced and educated professional programmer I should probably have a decent sense of which kinds of instructions are time consuming and which aren't, how the cache works, etc. Abstractions are leaky. You should strive to understand where your black box is transparent.