Q:

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

Do we need to reboot after adding an user to
/etc/sudoers

A:

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

A:

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.

