I forgot about the stock concentrate - I do the same as Judy - I reduce the salt in the concentrate by about 1/3, and I keep it in the freezer. As Judy says, it is still soft enough to spoon out, and there is no problem about it going off.
The only things I preserve by bottling first and then boiling the jars afterwards is fruit, because the fruit is fresh hasn't been cooked first at all. I figure with jams, sauce, chutney etc, the mixture has already been cooked, and as long as the jars are sterilised and seal properly, it should all be fine.