Do you have a drop checker to check CO2 levels? I'd start at 1 or 2 bps and see how the checker responds. It depends on how efficient your diffusion is too, and what method you use.
You're going EI aren't you? What substrate are you using? If you have a complete substrate, you might not need to dose straight away, but there shouldn't be any harm in dosing from the start as long as you have light as the limiting factor. Plants will only grow at the rate permitted by the light, and you won't have deficiencies to encourage algae.