I do not use such contaminated buffer in any preps I do. There is a short article published in Stain Technology, vol. Hope that helps. We routinely use cacodylate buffer for glutaraldehyde and osmium because it gives a finer grain than phosphate. Do remember that if you use uranyl acetate en bloc, you need to wash out the cacodyoate or the phosphate before adding the uranium because it is not soluble, and small precipitates will make the image very grainy.
Some folks use aqueus uranyl acetate.
You can add a minute pinch of sodium azide to prevent unwanted growth in any buffer. I would hesitate to add azide as it is a potent poison and may affect ultrastructure - especially of mitochondria. John J. Bozzola Gernot, Cacodylate is probably one of the most cytoplasmic extracting buffers where as pbs or phosphate is better but must be replaced before osmication. My rule is PBS or phosphate buffers for immuno-EM and cacodylate for standard fixations.