YES. I tip at least R5 and sometimes up to R100 depending on service.
The only time I don't tip is when I have no cash on me - which has been infrequent since I try and make sure if I fill up I can pay the tip.
I'm amazed at how many people don't tip. For those of us who can fill a tank with fuel a couple of times a month what is a few bucks to a low wage earner? And I challenge you to tip R100 next time and see the gratitude which will tell you exactly how close attendants live to the poverty line (when for most of us we wouldn't miss R100 a couple of times a month if it accidentally disappeared from our bank accounts).
Is MyBroadband going to publish survey results? It should be fascinating.