The thing about the TOEFL and IELTS, speaking good English doesn't necessarily translate into a high score. You could have a native English speaker take the test and if they don't know HOW to take it, they might struggle. Average/good spoken English won't help you if you don't know how to look at multiple graphs and write a report about them, or if you don't know how to write a proper academic essay. That said, it all depends on how high you want to score in the IELTS. Some universities want a 5 and above. Others want 6+. Some 7+. If your aim is the bare minimum (5-5.30 out of 9), walking into the test blind will do. That's assuming your English (spoken and written) are average. A notch below fluent.
Now if you want to excel and not just do okay, a bit of preparing might do wonders. The listening part is the easiest, unless you have trouble with different dialects being thrown at you. The reading part is progressive in difficulty. You'll get 3 articles with the last being the hardest. That means if you waste too much time on the first or second, you won't have time for the hardest one. They intentionally use words they expect the average test taker to not know/understand, especially for the third text. Mostly to see how well you handle 'meaning in context'. The questions/answers for the Reading section usually follow the same order of the article's paragraphs, except for some questions that require an understanding of the text as a whole.
The writing part is a bit touch n go for some. You'll need to write a report on some chart or graph for question one. If they ask you for 150 words and you write 149, you'll lose some marks. You'll need at least 150. When you report you CANNOT add any information that's not in the given chart, and you cannot include your opinion about anything. You also cannot exclude ANY information shown on the chart/graph. Same for the essay. At least 250 words, and you must answer the question that is asked, i.e. "do you agree?"
If you already know all of this, sorry
These are just some pointers and you can find more of them online and in youtube videos if you're more of a visual learner.