Tipping is done, but when I've asked servers and restauranteurs about it, they say it's not a "must". Here it's more of a courtesy. I know people who will only leave the last few cents of their change, and people who will tip 15%. But most people tip around 10%. What I normally do is figure out what 10% of the bill is, add the two figures together, and then round it up. So if the bill came to say R152.50, that's a tip of R15. You end up with R167.50. So I round it up and leave a tip of R17.50, making my total R170.
Are you only staying in Pretoria? Cause Jo'Burg has some great places too. Definitely try to make it to the Apartheid Museum if you get to Jo'Burg itself.