When Do You Decide What Rating to Give a Book?

Every time I finish reading a book, I rate it on Goodreads. I like to look back over books I've read, and rating them is an easy way to see which books were stand-out reads for me. It takes a lot for a book to be given a five-star rating, but there's a fair amount of leeway between three stars and four stars for me. This is often the hardest decision - I liked the book, but did I like it enough to give it a four-star rating?

There are two books recently where I realized I was thinking about my rating while I was reading the book, and that thought process was the same for both books. When I got to the middle, I figured they were each solid three-star reads for me so far. But I knew that whatever the ending was could bump it up to four stars for me. (The two books, in case you're curious were Moloka'i by Alan Brennert and Uprooted​ by Naomi Novik. Moloka'i ended up with three stars; Uprooted with four.)

The very fact that I was having this thought process while reading surprised me. I hadn't realized I thought so much about the ratings. And then it made me curious. At what point in your reading process do you decide what rating to give a book? Do you wait until you're completely finished (which is what I thought I did, but apparently not always)? Can you tell by the middle what rating it will be? Have there been any books that you thought you knew how you were going to rate it, and then the ending changed your opinion completely?

​Please add your thoughts in the comments. I'm really curious to know how other readers approach this!