You can read the Molecular Probes Handbook at Invitrogen's website for further information on DiI and other fluorescent probes. It is a great source of information if you work with fluorescent probes.

As far as I know, GFP does not leak after fixation. It is a (large) protein which will be cross-linked by the aldehyde to other cellular proteins. However, you must be careful with solvents, as acetone or alcohols can completely ruin GFP fluorescence.

As for DiI, it is not fixable with PFA. There are fixable DII derivatives, with a chloromethyl or sulfoni acid group attached to the dye. The dye should not leak from your cells, though, unless you add solvents or detergents. Last, even if the dye does not leak, DiI is very easily photobleached, so you have to be extra careful in protecting your cells from light.

Hope this helps,

Andre.

I tend to see leakage over time in embryonic labelling even with fixable DiI - which means I need to image right away - I imagine that if your results would be affected by leakage then as a reviewer I would ask for a control showing that no leakage is taking place. Depending on your experiment, you could also get DiI to pass from cell to cell before you fix - not sure it is ideal for proliferation unless you were to do a bunch of controls. The times I looked at proliferation I chose to use things like BrdU (although you would need to get around the issue of DNA repair, but you can get around that again with suitable controls). Good luck!