Do I need to wash the flax and sesame seeds before grinding them?
Yes, they're all ready to eat.

Washing them is just going to get them wet, and it'll be difficult to impossible (especially for the flax) to get them dry enough to grind to a powder.

People do sometimes soak flax seeds, or mix ground flax seeds with water, in order to get something to use as a vegan egg substitute, but I don't think that's what you're going for.