Narration is just a tool like any other in storytelling, in a VN medium you should like,
@DarthSeduction says,use the images to their fullest, just like you would expect in a comic book, however it isn't just a visual medium with words attached, like for example a play script, in which direction would take the place of narration, although that seems to be what
@DarthSeduction is getting at, effectively giving good stage directions to the render monkeys.
It is neither a purely visual medium or a purely novel/written medium, i wouldn't expect to go an watch a play with 50%+ narration, or even 10%+ however if i wanted to read a novel i'd expect a lot of prose, hell some of my favourite novels, terry Pratchett springs to mind as the most obvious, uses a heap of what amounts to narration and the style of said narration and prose are a large part of what makes the book great.
So what i'm trying to say is "Narration Bad!" is IMO limiting yourself too much, use narration but don't
EVER interrupt dialogue with it, it wrecks the flow, destroys the "feeling" in the scene and disconnects the viewer. Instead use it in the beginning to set the scene if you want, use at the end to explain time jumps or tedious journeys, actions or other "setup" stuff, use it in a scene to bridge two pieces together, use it to fill the holes a purely visual medium can't or explain things that are beyond the characters knowledge that prevent thought explanations. A VN is neither a play script, book, comic, game or film but can use elements of all of those, IMO narration is a valuable part of this, however it is or IMO should be a supporting part only, intended to facilitate the visuals, dialogue and pacing of the VN.
Just my two cents, i don't think there is a perfect answer here.