Whilst the instrument time is not a 'requirement' - as I understand it all 'responsible' schools do a small amount as part of the syllabus.
Given that 'nobody' manages the absolute minimum of hours (indeed, many have already logged 50, 60 or even more hours before their checkride) if some of them were done at night, then they will count towards a night rating...
And even if not, I do think it makes sense to get the night rating immediately (it is worth noting that the FAA system has no proviso for a private cert without night).
Cheers, Sam.