It is not necessary (all cell populations have a baseline amount of necrosis and apoptosis depending on culture conditions) . Just fix cells with formol (2:1 formol PBS; after incubation eliminate fixing medium by centrifugation, and incubated with colorants the same way as with samples) for 10 min and incubated with Annexin or IP. 100% of cells should be positive each control. Also need an autofluorescence control. Use cells without any colorant for these purpose.