Using all squares of the grid is not a rule of Numberlink.
It is OK if the Numberlink puzzle have unique solution with empty grid. See puzzle 2510 by Valezius.
http://puzzlepicnic.com/puzzle?2510
I noticed that in the Numberlink puzzles I've done so far, all squares of the grid are being used. Is this a rule of Numberlink?
This is an old type, and it was on many WPC. Normally this isn't a rule.
I encountered with this rule in not too good logic newspaper, so I think the WPC organizer's opinion is more relevant for us.
I think it is a totally unnecessary rule. So I'm happy that PuzzlePicnic didn't use this extra rule