You should post your code especially if you're unsure about it!
If someone sees something to improve and leaves a comment, they'll probably say what you should do instead, perhaps even why. That's pretty much the perfect way to learn!
I don't think it's likely they'll leave mean unconstructive comments, but if anyone does you can safely assume he's an asshole and can be ignored.
If you're really worried you can make two Github accounts, one for code you're more confident about and will include on your CV. But I wouldn't recommend it - you'll always be unsatisfied about code after a few years, that's not exclusive to juniors.
(Practically speaking, it's not likely that many people will look at the code close enough to leave any remark.)