For some odd reason, the Kindle default fonts (PMN Caecilia LT, Helvetica Neue LT and KindleBlackboxC) support all Greek letters in the
Greek and Coptic and
Greek Extended) Unicode ranges.
I.e., if you create a .mobi book with Greek text, you won't have to embed any font at all, as long as you make sure that all Greek characters are encoded as Unicode characters. You also don’t have to use HTML entities; simply copy the Unicode characters into your text.
If you plan to use accented Greek characters, you'll need to embed a suitable font for the ePub version, since ADE only supports basic Greek letters.