grid line is necessary for CS academic paper?
There are no such rules. I have seen a wide, wide variety of formatting styles for figures in CS conferences and journals. 

The goal is portray something to the reader. What provides them that information in the easiest way possible? Too many grid lines and they get in the way, but without them it might be hard to eyeball the y-value. 

I often end up making 20+ versions of each of my figures before finally settling on one that is most readable.