Is there a standard angle to use for drawing and drafting tables?
It hugely depends on you. Typical setups are around 30-40 degrees, but personal preference is paramount. Find an angle that you are comfortable with, and stick with it.

Personally, I prefer a flat desk, but some people I know aren't happy unless it's closer to 45 degrees.

Bear in mind that whilst learning, some teachers will encourage you to change your angle if you are having problems with distorting your proportions. It will force you to look at the drawing differently whilst you draw it.