My designer tells me white lattice is dated and I should paint it sage, the color of the house. I like the "pop" of white. Is white lattice passe.

If you like the pop of white, by all means, go with it! There are so many different tones of white that you can create a really great mood or nod to a style of architecture or decor. And keep in mind that you will very likely have lush plants growing up and around and attaching to it. Think longterm what you want it to look like. Thanks for you question!