If I'm just making it for myself, I don't always add a preservative. It depends on what the cream is going to be used for (i.e. body vs face). I know I'll use up a batch of body cream far faster than I will a batch of face cream, so I'll put a preservative in the face cream, but not the body cream. If I'm making it to sell, I add about 5 ml (1 tsp) of germall to the cream.
As for separation, yes, I have had that problem occasionally. I was going to ask Ananda about it, actually. I make a knitter's hand cream that separates a little. BUT.. I think I've managed to fix it by replacing some of the beeswax with emulsifying wax and cutting back slightly on the liquids. Seems to be better now. I think the problem is that there may be just a little too much liquid compared to the oils. I don't think you HAVE to use all the liquids. It may even depend on altitude, temperature.. all sorts of things (just a thought).
As well, instead of waiting for the whole oil/wax mixture to cool completely, I now put it in the blender while it's still somewhat warm, whiz the liquids in and then wait until it cools completely, whizzing it now and then. That also seems to help.