For trains without required reservation, do you still recommend it?

That depends on the train, the day, the time of the day and with how many you're travelling together.

You canNOT get on all the time-but say in >99% of times. But about each&any newbee on the rails seems to want to ride exactly those trains that are oversubscribed. Very busy days should be evident with a tiny bit of thinking: start and end of major holidays and long weekends.