As with many other developing countries, staff in the South African service industry are probably making just enough to put bread on the table so tipping is always a good idea. If you feel that the service was sub-par then don't.

Car guards are for the most part organised beggars and wouldn't do anything to stop someone actually stealing your belongings, for one they aren't going to risk getting stabbed by the would be robber for R5. If you park somewhere regularly it is a good idea to tip as not doing so might increase your chances of returning to a damaged vehicle. A few years ago I parked just off Long Street in Cape Town and only tipped the beggar/guard R2 (he wanted R5), he made it clear that my vehicle would not be safe if I parked there again.