Do we need to reboot after adding a user to sudoers?

No. It'll work with the next sudo command.

But if it does not work, you can avoid rebooting by running

sudo service sudo restart

I just did this and yes, I did in fact have to reboot. So, maybe the previous answer wasn't wrong, but it definitely isn't right 100% of the time. Writing this in case someone else is looking for the answer as I just was.

In CentOS 7 you can also Logout from the system with "exit" and Login again and the Sudoers will be updated.

I've tested on minimal installation but I believe it works in other targets and possibly other distributions aswell.