This, but don't over do it. Flex is based on you, not so much the type of skiing you do. Your boot should be supportive and powerful (efficient) yet not so stiff that you cannot flex it properly (not able to get your knee over your toe).
Boots that are too soft cause just as many, if not more, problems as boots that are too stiff. Find the right flex for your body weight, ankle mobility, and strength and you will be golden.