I think Attila123 knows best. At first, I'd say that at SIN, you could literally arrive at 5:15 and easily make it--especially without luggage, since there are NO flights leaving at the same time as NW (in the whole place, I think NW is the first AM flight, so you won't encounter trouble at immigration). Security is, unfortunately, at each gate, so you have to wait there.
However, at least when the 742 was flying that route, Op-Ups seemed to be done early, rather than later: The 2 times I was Op-Upped ex-SIN, I was on WorldVacations packages (so in a super-cheap, group ticket class) with an ungodly airport shuttle departure time: 2:30AM! As one of the first people checking in, both times the agent saw my PE card, called over her boss, and they Upgraded me to WBC. I know there are many fewer WBC seats on the 332, but I will say that I have had luck on early check-in at SIN!