The lab claims fixation will lead to the dyes to leak from your cells. I think the belief is the fact fixation may possibly harm mobile membranes. I did look for a paper which describes fixation employing methanol or acetic acid, just after staining that has a DCFDA derivative. DCFDA https://2-fdckhcl13456.webbuzzfeed.com/29882838/the-smart-trick-of-2-fdck-hcl-that-no-one-is-discussing