Providing you are staying at a nice hotel and have reputable transfers, I would not worry about this.
If you are overnighting in Joburg, you may just want to stay close to the airport at someplace like the Airport Grand Hotel on the budget side (under $100 USD) or at the Airport Continental, a 5* hotel.
My favorite Joburg hotel is the Michelangelo Hotel in Sandton. The good thing about this 5* hotel is that it is located in Sandton Square and this is pretty much a self contained little city in itself. This is perfectly safe to walk around, day or night, in Sandton Square and there are many restaurants, shops and even a live theater (the Liberty Theater). The best Indian food I have ever eaten, Bukhara, is located just across from the hotel.
Finally there are a couple other very highly regarded hotels in The Grace and Ten Bompas.
http://www.legacyhotels.co.za/content.aspx?h=6http://www.ichotelsgroup.com/h/d/ic/1/en...elsearchresultshttp://www.thegrace.co.za/http://www.tenbompas.com/http://www.bukhara.com/Bottom line...if you stay at a nice enough place, a night or even two can be very rewarding in Johannesburg, especially prior to a safari so you are recovering from jetlag at hotel rates rather than at game lodge rates.
