You should class supports on time ~ all nations. The exception to this would be is if you had some plan about your end goals. I've seen a player go with all engineers on their UK by late classing their engineers so they would provide extra repair. But they had a long term plan in place and weren't using repairs. The point here is that you would have a totally different assortment of crew and a purpose to what you are doing.
Another easy example is with torpers. Torpers reach their cap quite easily so I might leave them as national sailors for a long time so they gain repair and restore ability while sacrificing their torpedo ability. In the short term, I would be giving up torpedo ability but in the long term, I will have full torpedo capability along with good repair/restore. Once again, it's a matter of looking at the end product. The most likely instance of late classing is with sailors that cap easily since the main ability of the end product whether classing on time or late are the same just that the latter will have more support stats.
I guess this goes back the Eufo's original response of ~ if you don't know(or have a plan) about classing late, then you should be classing on time because supports should be classed on time. I've also mentioned previously that if you don't class on time, your total crew will be less. At each classing, you can add more recruits ( thus the potential for more experts/vets).