LAST EDITED ON Apr-25-13 AT 08:58AM (MST)[p]March-April is a little early, but definitely do-able. The bush will be thick, the weather warm and you'll have a chance of some rain in March. If Kudu are a primary quarry, mid-May is an excellent time to go as they are in rut and the bulls are just as dumb as deer or elk that are rutting.
Of the four safaris I've taken, The first one, in Zimbabwe, went the last 3 1/2 weeks of May in Deka and was the best hunt I've ever been on, period. I did my next in late July to early August up in Charara and it was good but much more challenging. After that, hunted most of the month of June in Botswana, both the Kalahari and Linyanti Swamps and it was very good but high water made hunting the swamps more challenging than normal. My last trip was in 2009 and we went the last week of May and first week of June in Limpopo. During the first week, Kudu were rutting but during the second they were done and back to normal behavior.
Northern South Africa is a wonderful place in mid to late May. Where you'll be hunting, and the primary species are important in determining a 'best' time to hunt. Your safari operator will love it if you come in March or April as that is a tough time to sell hunts. Primary safari season in that region of Africa, if there is such a thing, runs from May to November.