You hear it a lot- if you want to write for a particular genre, you should read up on books in that genre. So if you want to write supernatural romance, you're advised to read supernatural romance. If you wish to write about time travel, they tell you to read about how time travel is done elsewhere. If you have an idea for a dystopian novel, you're expected to check out other dystopian novels first. The reasoning behind reading what you want to write is that you become familiar with the world and rules associated with a particular genre.
While it doesn't hurt me to read books from my writing genre, I strongly believe that reading in only the genre I'm writing in would greatly limit me as a writer. Not to mention I like making my own rules.
Reading stuff outside my genre actually helps me to reflect better on plot and character development because I get to take myself outside my reading comfort zone and drop myself off somewhere new where I might absorb details of the fictional surroundings more acutely than I would if I were in familiar territory.
Example: I write fantasy and suspense and happen to enjoy reading about certain teen wizards (no names!). But if all I read were books by authors about teen wizards where many of the conflicts depict characters hurling magical chants and shooting wands at one another, I would miss out on learning how conflicts are handled in a story without magic. Reading a magic-free story about the new girl who uses her wit to take down the school's queen bee, I learn how the emotional arc evoked in a conflict-to-climax moment yields the same amount of tension and page-turning frenzy in both fantasy and non-fantasy writing. I also learn that it's not the chants or wand magic or the queen bee's secret fear of heights that determines the outcome of a conflict. The characters steer the conflict to its climax and the story's outcome by their choices and their response to a challenge. The wand is just a prop.
My advice to anyone who wants to write in a particular genre is not to limit yourself by reading only in that particular genre. You grow when you branch out.