I've seen other questions regarding the same, but in this case I think it is a little different. I know the Markdown handles tabs correctly (when generating the HTML code), but that's not what I'm requesting.
I think it would be good to automatically convert tabs to spaces when storing the code in a post. I mean, changing the Markdown source.
Although you think nobody mixes tabs with spaces, well, lots of people copy some piece of code from a website, or from their IDE, and do some edits (maybe looking for a correct indentation here on Stack Overflow), and then it happens.
The thing about it is that it makes editing much harder. You have to copy the code to a text editor, replace tabs with spaces, and then paste the code on the site again and indent it correctly.