Here's a question that seems to worry students, especially those in China:
"A famous IELTS book in China tells students that they should write 340 words or more (for task 2) if they want to get band 6.5 or higher. However, other tutors tell us that we should use the 10% principle, which means we will lose marks if we write anything more than 275 words. Which advice is correct?"
Here's my answer:
All of that advice is wrong, and I don't know why teachers invent these things! The only rule is that you need to write at least 250 words. So, whether you write 250 words or 500 words, it is possible to get a band 9. As long as you reach 250 words, the examiner only cares about the quality of what you write.
PS. I advise my students to spend 10 minutes planning before they write anything. This forces them to focus on quality rather than quantity.