RAW: characters are stuck with at least 5 Obligation.
Not quite.
RAW: Characters can never have less than 5 Obligation. There is a subtle - but important - difference in that.
Saying that there is some nebulous 5 point Obligation that lurks at the base of every character's existence is not RAW. No, telling your players that they have "Life Sucks: 5-point Obligation" is indeed a house rule.
Rather, it is the GM's job to make sure a character never manages to wipe his slate clean. That the process of working out that last 5 points of Obligation inveriably results in something else popping up to take its place.
They're actually quite exactly the same thing. No matter what, the PC can't ever *actually* pay off their Obligation pool in full. That doesn't mean that a PC couldn't pay off the last 5 points of their Debt Obligation, in cash or kind before something actually Obligation-worthy happens to come up to replace it. That means they have no more Debt, but they must still have 5 Obligation or more according to RAW.
Filling it in, for the time being, with 'Life Sucks: 5' is no different than immediately dropping 5 points of any other Obligation on them, except that it doesn't have one of the names in the Obligation table. (Hint: Those are only a list of suggestions anyway, not the full and complete list of possibilities.)