No. A lot of people say that but there's no quote stating it. George Lucas has gone on record to say that the Sith throw balance out of whack. A rumor went around for awhile that I, admittedly, believed in which Lucas stated the Dark Side is a cancer in the force. People sourced Bill Moyers and the Mythology of Star Wars. Niarcromon brought it to my attention that this isn't true. I went on to watch it myself and nowhere does Lucas state that. So it seems the only thing that throws it out of balance is the Sith themselves which makes sense considering the various dark side cults don't attempt to conquer the galaxy and pretty much do their own thing on their own worlds.
I could be wrong on this but I've never seen an actual source where Lucas states outright that the Dark Side itself is imbalanced.
Hmm. I guess your right, I too havent seen a source, but it seemed so believable. Considering lots believed it and, it seems like someting Lucas would say. Well, Niacromon brought it to you, and now you bring it to me. I see.