You need to know the current era is Heisei and the previous era was Showa. Knowing the years (even roughly) would help.
Beyond that, it is nice to know there was a Taisho and before that a Meiji. Knowing the years is nice, but not essential. For those eras it is enough to have a vague idea and to be aware of what state of development Japan was in.
Anything prior to those, don't worry about them and just look them up if necessary. I have a free app on my iPhone which is very handy when I need to look up older dates, which I do fairly often as one of my hobbies is hunting up old religious statues and tombstones. The dates on those are always one of the older eras that nobody bothers to memorize.