This has been discussed here on the forum a couple of times and if you do a search, you should be able to read some differening ideas.
However, after much thought this is what I have decided;
If the holster is just about beyond hope, i.e. beat to death, dry to the point of ruin, then I will apply something such as Hide Food to it, otherwise, I clean it gently with warm soapy (not too soapy or too warm) water and let enough alone with it.
Ed