Gilly bee,
There should be enough stores in a nuc for a couple of weeks. If it is a proper nuc.
There should be as much forage around that they can collect their own - that is what the foraging bees are for!
A nuc should be keep warm and cosy with just a frame or two of foundation to draw as the colony expands.
The nuc was likely not being fed extra before it was purchased, so what has changed?
A nuc is only a small colony, so should expand quite quickly in the right conditions.
NOW, if the weather is unfavourable, the nuc is not a 'balanced' small colony (produced just prior to supply), is short of stores, etc - then yes you feed. As I said, if there is a reason, yes; if not what good does it do? I hate words like 'invariably' as it doesn't leave any lattitude for discretion.
I would not just simply transfer a 5 frame nuc to a box where they only occupy half the frames in adverse conditions. Just cr*p beekeeping, if you want them to expand quickly.
Regards, RAB