Note that the most commonly used ePub reading app, ADE/RMSDK, ignores
all guide items.
OTOH, if you want to design ePubs that can also be used as source files for Kindle books, you'll need to identify the cover, the HTML TOC and the SRL (start reading location) using the Semantics menu.
If you apply these three semantics items correctly, the guide section in your
opf file should look like this:
(The
href paths are of course different in Sigil.)