Hi Dean,
It isn't necessary to obtain an IMC before a Night Qualification, I had a Night Rating (as it was called) before obtaining an IMC rating.
However back in the good old days you required to have a certain amount of instrument time - 5 hours if I remember correctly. Quite sensible as you're flying without a horizon at night where you could end up in clouds that you can't see as easily as in the day time.
Also if youi're flying out of a Class D airport at night without an IMC rating you will need to take a Special VFR clearance out which a PPL/Night will legally need 10 kilometres visibility. With an IMC rating you could take it down to 3Km or if you have a suitably equipped plane fly an IAP with 1800 metre vis.
At night, as the atmosphere cools down, the chances of the visibility reducing to under 10km is quite possible. So what would happen for a PPL/Night flying around the local area finds that the visibility has reduced to below the 10km requirement for the Special VFR? Hmmm... not many airports outside controlled airspace are open at night.
So I would most definately say it is very wise and sensible to obtain an IMC rating before a Night Qualification.
Best wishes,
Charlie Zulu.