Although I'm at a high English level, I still translate a lot. Sure, ideally you would look up the words in a monolingual dictionary. I bought this big "Oxford Dictionary of English" and am now checking the definitions of difficult words there first, as often as possible. But translating them into German wins most of the times, just because it's quicker and I'm more likely to understand the words correctly. :) So I'd say a "reasonable mix" would be best.