The main purpose of rating anything is to keep a record for yourself of how you felt about a particular movie, anime, album, or basically anything that you can have an opinion about.
Rating is ranking
If you rate something a 10 you are saying that it means more to you than something that you rate a 5. And if you rate something the same score as something else you are saying they are equal.
For this reason I sometimes gave ratings with decimals but I’ve stopped now because it made things complicated and it was pretty inconvenient.
Quality vs Enjoyment
Quality is enjoyment. Your score should reflect how you felt about a show. Even if it had terrible production or other technical aspects, if you enjoyed it you should give it an appropriate score.
On the other end, you shouldn’t feel obligated to give an anime a higher score because “it has great animation” or something like that. Unless, of course, great animation made the show enjoyable to you. Again, it’s all up to you because you’re rating for yourself— not for others.
Bad vs Good
I would consider a bad anime anything below 5 and a good anime above 5. An exact 5 would be an anime that falls directly between those two.
A 6 or 7 is something that I enjoyed but probably won’t remember or care about after a while.
An 8 or 9 is an anime that I thought was great but just failed to get the top spot.
And then we come to the 10/10.
The perfect score
Ironically, a perfect score doesn’t necessarily mean the anime was perfect. Rather it means that it had a meaningful impact on you. It’s something that you won’t stop thinking about for a while after you finish it. It’s the kind of anime that you don’t want to end because that means you won’t get to see more of it.
Revisiting old shows
Your tastes are always evolving and sometimes you may not feel the same way about something you saw a couple years ago. In this situation it’s okay to change it.
However, you may also want to leave it as is as sort of a rating of the memory of watching the show and not the show itself. If you watch that anime and it brings up all the great feelings it did the first time it’s fine to just keep the initial rating.
I’m constantly changing my scores. I’ve changed almost every score I gave to this season’s anime (Unfortunately it wasn’t positive). When you’re in the moment you may give an undeserving score so it’s best to spend more time thinking about it to get an accurate one.
Explaining why you gave a score (whether writing or speaking) usually helps to organise your thoughts.