It isn't a 'sin' but why do you think it will be helpful?
Please feel free to avoid the next bit if you are bored by educational theory
You can mark kids work in a vaiety of different ways.
You can do the old style 75/100, came 5th.
You can do the modern, Attainment A, effort D stuff.
Or you can just correct the errors, tell the kids what they have done well on and tell them how to improve, so for example, 'You made a mistake on the calculations, you understand density well, to improve, go over the calculations remember that density = mass/volume'
Trials in classrooms show that children do best when they are simply told how they have done and what they need to do to improve.
They do least well when they are given marks and a position in the class. Why is this? One of a few reasons. Firstly knowing that you are 5th in the class doesn't help you to get any better. Indeed, coming top may make you complacent and possibly lazy. Comming bottom, especially for kids who work had may end up with them switching off and feeling that school isn't for them.
Even being given grades can have the same effect. The bright kids with As for attainment and Ds for effort know that they are baright and that they don't need to work. Not so bright kids who get Ds for attainment and As for effor end up thinking 'Why bother trying?'
Ultimatly wht matters is that you child makes progress and reaches their full potential. Knowing where they are in the class isn;'t going to help this one little bit. As other have said they could be top of the class in a bottom set, or bottom of the class in a super smart set. What does it tell you that helps them to learn?
Nothing. And it can be harmful to their self esteem, and work ethic.
People interested in this sort of stuff should google 'Beyond the black box' and formative assessment.
Amazing as it seems, teachers do sometimes have good reasons for doing stuff