probably
If you have many tabs open you probably can't see them all in the tab bar. And, I'm not aware of any way to see what they are, other than stepping through them with Next/Previous Tab. If its easy to do, it may be useful to have the Book browser italicise the file names that are open.
I wish Sigil would allow no tabs - i.e. a blank canvas.
Other tabbed applications permit no tabs to be open; e.g. MS Office (via an add-in), Firefox, text & image editors, my PDF viewer, and the calibre editor - and I've customised them all to use Ctrl+W to close the current tab.
So to 'clear the decks' I hold down Ctrl+W to close all the tabs - except in Sigil it is all bar one
BR