I'm with AnemicOak (post #4) and exactly for the same reasons. Yes-No-Yes.
I had my first look at html because I wanted to make changes in the books. Now I can throw some pages into Sigil and get a simple epub out.
I almost never look at the code before reading, except when I know what to expect. For example in case of series - they tend to have similar formatting, so when I'm fixing #1, I'll check the others too. Or PG downloads where I add indent and remove space between paragraphs. After reading I fix all the bookmarked places and change for example poetry to smaller font etc.
Maybe I'm control freak, but I don't trust Calibre conversions - I want to know the changes I make. It's so easy in Sigil, and changing css takes less time than trying for right conversion settings in Calibre. I do use Modify Epub plugin.
Even thinking how Calibre adds some code during epub>epub conversion bothers me to no ends, so I avoid it. But crappy code doesn't bother me so much that I feel compelled to fix it if the book looks ok. Though sometimes I succumb to the temptation with the excuse that I need to learn. Depends on the book - how long it is, did I like it, can I be sure that the code is not needed....
So ideally, I'd like to fix the crappy code, but don't know enough to always recognize it or skill to fix it quickly.