You do not necessarily need radio triggers. Canon’s optical wireless system does not always require a direct, unobstructed line of sight; the slave sensor just needs to detect the triggering light, which can sometimes bounce off nearby walls or pass through/around an umbrella setup. In practice, flashes like the 430EX/580EX can often still fire reliably if you orient the sensor so it can “see out” toward the room or a reflective surface.

That said, with umbrellas, softboxes, and other modifiers, optical triggering can become unreliable and may require awkward flash positioning. Indoors it often works better because of reflected light; outdoors or in less reflective spaces it can be hit-or-miss.

So: optical control can work with modifiers, but radio triggers are often the more reliable choice if you want consistent firing and easier placement. If you rely on TTL, you may still use the camera’s optical system and accept its limitations.