Hi!
This is a good question. The Amp VU was intended to provide some indication of the level going into the effects rack. Whether you need this or not is questionable (I don't think I ever look at it). The devices in the rack are all floating point so there is no concern of headroom and clipping, however, the Delay unit does include a saturation model, so the harder you drive DelayThing, the more saturated the delays will be.