This is a question that may kick up a lot of dust...I've heard about people said every extra once on the ski is beneficial for them bombing down the hill because of the extra stability. I've also heard people said unless a pair of ski is too heavy or too light, they normally would not be able to tell the difference, and weight does not affect the ski's performance as much as the design (profile, construction, materials, etc). IMO it is purely personal experience. I like a pair of lightweight ski since they are easier on the legs and knees (I have Vwerks RTM, they are almost the lightest skis in its category) and I cannot say a pair of much heavier skis can give me much more confidence in term of stability at the speed I like to travel. But hey, someone can just barge in and tell me I'm absolutely wrong. So, again, it is purely personal. The "ideal" should be unique to oneself, not a general standard.
What I normally do is to demo skis until I find several pairs with characteristics that I like. Then I'll pick the lightest one lol (or the one with the best looking top sheet)~
BTW there are also some skis in the market that are light and stiff (Vwerks Katana), and also ones that are relatively heavy but soft. It really depends the design more than anything else.