There is a place for saving space. I well remember optimizing a number to character conversion routine to save 3 or 4 bytes off of a 20 byte subroutine on a computer with a whopping 16 kilobytes of memory (circa 1980).

But given that my Kobo ereader has 3 gigabytes of memory (that's 187,500 times as much as that 16K machine) I also don't worry about a bit of extra size in an epub file. A quick perusal of my font folder shows 161 files only 60 of which exceed 400K. The remainder tend towards 100 to 200K.