I have never heard that and I can think of absolutely NO reason whatsoever that it would make a difference whether you were vegging and flowering. We keep pH levels within certain parameters to facilitate the uptake of nutrients. While these levels are different for soil and hydro, it doesn't make a difference whether you are vegging or flowering. Nitrogen, which promotes good vegetative growth, would not even be accessible to the plant at pH levels under 6.0.
Anytime, I read things like this that go against the normal thinking, I want a scientific reason why it would be so. Do you remember where you read this and whether they had any explanation of why this would be?