Its actually not a need. Its on the person's free will, but its good to notify he can do so. And in case you do not know how to +rep, you can do so by clicking on the balance at the top right hand corner of the post. Approving is +rep, Deproving is -rep.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory
I will be AFK for a couple of days you guys hold down the fort while I am gone. I will be checking in on my phone but that will severly limit me. Be back saturday!