Idioms can improve your score if you are using it in the IELTS speaking test but it can also lower your score if it is not used in the proper way. Usage of idioms will be different between the native English speakers as well as the speakers who have English as their second language. Idioms are usually used in informal speaking. Hence, it is advisable not to use idioms while taking a writing test. To know more about the idioms of the day you can visit this website.