Thanks very much Idolse for your detailed response. Certainly looks worthy of tinkering with, have never tried that setting.
I know I am at risk of going O/T with Calibre discussion in a Sigil forum here but this is all related to recommended ways of conversion to make the Sigil editing work easier...
One thing that I have found with Calibre is due to the way it stores the conversion metadata I have to be careful to "unselect" stuff when doing different conversions. i.e. I always want EPUB to be my "master copy" since it converts so easily to other formats. So the first conversion will be from something else to EPUB for tidying up in Sigil. After that I then need to convert to MOBI for use on my Kindle. However I found I need to make sure I deselect any Calibre conversion options before I do the EPUB->MOBI conversion or else some of my careful Sigil work gets undone.
Is this what you would expect or am I doing something wrong? Because of this I don't really set much in the way of "global defaults" for conversions since so many settings are common to all formats but you actually only want them to be applied to the first conversion. The "re-run" factor to other formats becomes an issue when you turn these things on. Maybe I just got unlucky or imagined it...