"The more idiomatic expressions you use, the better speaker you are" ...hmm, no not really. How often do you apply idioms in Turkish? More importantly, WHEN? The real indicator is knowing when to use those idioms.
Here is some food for thought: the moment you open your mouth and the (native English-speaking) listener realises you are not a native speaker, chances are that the English speaker will avoid using idioms with you (so as not to confuse you).
It really depends on who you are talking to; though in general, you should be able to handle idioms quite well anyway.