I always stop at the end of a chapter. With rare exception, and no matter what I have to do to get there.
Where I have to watch myself... when a book is so good that I read a few lines into the next chapter and must then finish another chapter. What ends up happening is an overly dramatic book-slamming-shut moment when I know I have to stop reading. You know those times. Have to pick up kid from school. Must make an appointment. House is on fire. I know I must slam the book shut before I can read a few words into the next section.
On the rare occasion that I have to stop short of the next chapter, things like a child screaming in a must-take-notice sort of way, I will put a bookmark in and mark where I stopped with a pen. But, in those cases, that screaming child better be hurt. I'm just kidding. Well, mostly.
Can you stop reading anywhere?