If you put them directly into liquid Nitrogen the cells will most likely lyze (burst). Ideally, gradual freezing is best because it helps cells to freeze slowly not damaging the membranes too much. They even have automated machines for doing that when it comes to saving blood cells for a later infusion.
In my experience though, any type of freezing cells ends up in losing a bunch of them. So it is worth doing it as carefully as possible: first on ice, then -20 for a couple of hours, then -80 overnight, then liquid N2.
When stored at -20 degrees, some biomolecules will still be degrading.
I would suggest fixing them instead, for example, in formalin.