If you're using the -um- , always use "pwede" or "pwede ba" to soften the tone.
If you're not using that, use paki- + verb.
The only context -um- will be accepted is if you are a parent/grandparent and you are commanding your child. Outside of that, it's usually considered rude and aggressive, especially if you are speaking with someone older or someone who is not familiar to you.
Pwede bang pumunta ka doon sounds polite but pumunta ka doon comes off as really harsh. It comes out as if you are scolding the person you are talking to.
It's more of the cultural connotation than the grammatical aspect.