Whenever possible, yes!

Dead cells cause nothing but trouble in flow cytometry data - they're sticky (and so bind your labelled antibody non-specifically) and tend to have higher inherent fluorescence (aka "autofluorescence"), leading to false positive signals.

Viability dyes such as propidium iodide (PI), 7AAD, DAPI or fluorogold allows you to discriminate between live and dead cells and thus ensure only viable cells are analysed or sorted for further experimentation. The only time you shouldn't use a viability dye is if you have no fluorescence channels to spare for this in your assay.