Just got a pair of R-305 subs. Am I correct to disregard the yellow wire and hook up the red and black ones to each speaker terminal?First, I assume that your amplifier is not bridged or fully balanced, meaning that the black speaker terminals are grounded in the amplifier and not driven with signals. As you probably realize, in the case of a bridged or fully balanced amp the black wire should be connected to amp chassis, not to the black output terminal.
Given that assumption, the connections you are describing may work ok, but a concern would be that the "floating" yellow wire may result in noise or hum being introduced into the sub. What would be better, and would eliminate that possibility, would be connecting BOTH the red and yellow wires to the red amplifier terminal of the corresponding channel, and the black wire to the black amplifier terminal of the corresponding channel.
Connecting the yellow wires to the black amplifier terminals, along with the black wires, would also work. It would result in the subs seeing 6db less signal than if you connected the yellow wires to the red terminals, causing you to raise the settings of the subs' level controls correspondingly higher.
Finally, when you refer to connecting "to each speaker terminal" I assume you mean each output terminal of the amplifier. That is generally what is recommended, as opposed to connecting at the terminals of the speakers themselves.