For clothes, it is worth the time invested if you can go to the retail brick & mortar store and try on the item itself or at least something similar in cut/style. I love Banana Republic and I know my size in their dresses, tops, pants, jeans, skirts, coats, etc. Believe me when I say that the my size can range at the same store depending on if it's a top or dress, slacks or jeans.
I would recommend against buying from a brand you have never tried on before or a brand you haven't seen up close with your own eyes. For one thing, you have no idea if the sizes are made big/small and second, you have no way of gauging authenticity of an item if you can't compare quality/material/craftsmanship. I personally buy only a few key brands that I am familiar with because I can be 90% sure that the item will fit even if I've never tried it on, and I have a better chance of detecting a knock off.