LOL! Did your mother grow up during the Depression by any chance?? My mom always did this too ... partly because storing upside-down "seals out" the air-space that would otherwise be at the top (she did this with all her home-canning too) ... and partly because, in a forgotten age .. dairy products weren't mass homogenized. Fat and cream would separate out and rise to the top. Storing them upside down meant you still got a layer, but it wasn't congealed on top. Organic and natural products are non-homogenized (and taste better IMHO) and you get the layer of thick cream which you have to stir in ... It's either a pain or a delight, depending on your point of view

I think though, that once a container has been opened, it's best to store it upright ... since most bacterial growth will happen around the open edges and lid. You don't really want the food sitting in contact with those edges.

Doreen