To be honest its not that I feel that watching the anime spoils the vn. Rather that it really depends on how the anime was to the person watching the anime that will motivate them to play the vn or not.
Well, its more of a story of what happened with me and my friend when we watched the Tsukihime anime. It was last year or maybe two years ago that my friend talked to me about the tsukihime vn. I had already played through it and.... he wanted my honest review of the VN. To cut a long story short, he watched the anime before playing the vn and was really disappointed in the anime(me too).
I dont mind having watched the anime before playing the vn. Some people cant stand spoilers but I dont really mind. The feeling of playing the vn is also very different from watching the anime. So, I will play the vn even after watching the anime. Probably depends on whether someone can stand spoilers or not.