Given the massive installed base of Kindles that do not, and never will, support KF8 it is important that however you design a book it should look good and function well on non-KF8 devices and software. It will be quite some time before it's safe to ignore these non-KF8 devices.
KF8 files generally contain two versions of the formatted text, one for KF8 compliant readers and one for non-KF8 compliant readers. Both versions need to be tested.
Embedded fonts should be used only for special text (headings, titlepage, ...) and not for the main body copy. Even then, the book needs to still look good when the non-KF8 version is displayed with its fall back to the device's/software's own fonts. The same applies to ePub versions. Not all ePub readers support embedded fonts.