Hello,
You should use the vector font for any text which appears in the Gerbers, e.g. on your silkscreen, otherwise what you see in EAGLE might not be what you get on an actual board. Fortunately Autodesk added a nicer vector font in a recent version of v8 so it looks better.
In the schematic symbols use the proportional font as it looks better and makes it a lot nicer to read.
Best Regards,
Rachael