You can use a couple of idioms but only if you are 100% sure that your level of writing is really high. They shouldn't look inappropriate in the context. If you are not sure don't use them at all! It won't reduce your score, that's for sure. If you use them in a right way, they may make your score a little better, but it's questionable. 

PS. I got 8.5 in writing. I didn't use idioms.