I played 7,8 and 10 and while getting through the storyline isn't that hard without a guide, beating superbosses and finding secrets seem to be impossible without one. Someone on YouTube said that IX is full of missables,and as a completionist I would hate to miss anything cheesy.
Not to mention that I want to beat the giant swirly rainbow coloured ball of impending death, simply because it is supposed to be THE ultimate boss, and that isn't a challenge I can decline.
Can you, or can you not?