Quite often JLPT level 1 grammar and comprehension questions involve a variety of level 2 grammar. If I remember correctly, this also applies to some extent to levels 2 and 3. So it’s in your best interests to review the grammar points of the level below that which you are taking.
This being the case, I’ve just started reviewing level 2 grammar. I’ve forgotten quite a lot of it and considering the fact that I’ve been reading various formal and informal texts, this goes to show how little some of these grammar points are used.
Having said that, a lot of JLPT 2 grammar is very useful, more so than level 1 grammar, in my opinion. There is also more grammar in level 2 than level 1, so brushing up shouldn’t be taken too lightly.
I need to revise my level 1 grammar next. It’s definately more full-proof to learn the grammar thoroughly and score well if your reading comprehension is so-so. Of course, if your reading comprehension is fantastic then you’ll probably pass without having to score much at all in the grammar section.