If you're solving a lot of textbook problems for Chemistry Olympiads, it is advised not to do so. You might be inclined to think that there's no harm in solving all of the problems in a textbook like Atkins, but you'd be surprised that you could be doing more harm than any good.
• The type of questions in a textbook (i.e. Atkins) are very limited. They are very simple and straightforward. Sooner or later you will get bored and end up solving problems without thinking, which is a terrible habit to form as Olympiads like to trick you with questions that seem very similar to textbook questions.
• There is a limit in terms of effectiveness of homework problems. Solving 10-15 problems per chapter is often more than enough^[exception: if you don't understand the content or have problems getting homework questions wrong, you should probably re-read the content and then try solving more homework questions. The purpose of homework problems are to make sure you've fully absorbed the content, not for completion.]. As questions in textbooks are very similar, you'd be solving the same question over and over.
• It's always nice to practice a bit higher than your comfort level. This doesn't mean you should jump from textbook problems to IChO problems, but you shouldn't be afraid of getting a lot questions wrong on a USNCO local/national exam. Start practicing as soon as you've absorbed the fundamentals.