Often there is a delay between sample prep+staining before acquisition on the flow cytometer. Depending on the assay, the stained cells may be fixed and data acquired within 24 hours, but not all assays can be fixed in this manner. Some assays require the cells be fixed as part of the sample prep and staining, and usually you can acquire the data hours later. If you are performing a live cell assay that cannot be fixed, it is best to acquire the data as soon as possible; sometimes placing the labeled cells at 4º C may help. For live cell assays, I recommend looking at the data sheet from the manufacturer for guidance on compatibility with fixation and on storage before acquisition.