Every cytometry experiment should contain a viability dye. It doesn't matter if it's just analysis or if you're sorting.

No ifs, no buts.

Dead cells can negatively impact cytometry experiments. They can also skew your functional assays and even your everyday cell culture. Unlike some other experimental and cell handling procedures, the impact of cellular death is easy to quantify in cytometry assays, and as such should be factored into all your experiments.