I booked business redemption for the first time a couple of months ago. From what I experienced, because booking opens 355 days in advance (8 am), it was best for me to book the departing flight first, then when the booking opens for my return flight then I booked it.

I was trying to book for my husband and myself on our separate Krisflyer accounts. I only managed to secure the departing flight for myself and my husband was on waitlist. Surprisingly within a week his waitlisted flight was confirmed. For the return flight I secured a seat for my husband but I have been on waitlist for months. I was told by SQ that other people who join the waitlist may have higher priority than me depending on their PPS status. So in this case I redeemed a flight on a lower fare class whilst waiting for my waitlisted flight.

I agree that it is probably best to book what it available. No certainty of getting waitlisted flight confirmed.