As a builder myself I share your annoyance with the front panel connectors, why this cannot be standardize, I haven't a clue.
As for your questions,what case are you using? but I do not think it matters if they are not marked + or -. Thinking back to some of my cheaper cases I just plugged them in and hoped I had got it right. I haven't had a case for a long time that the cables were not marked.
Not much help I know, but hopefully others will give better advice.