I don't think you should ever be without an onboard booking "in the bank". What I have done is ALWAYS book something while onboard even if I have no idea or plans for a future cruise. If you don't have a specific cruise, just pick a cruise that is a long ways away (hopefully a cheap one with maybe even 2 for 1 pricing). A 10% deposit is necessary.

This booking is turned over to your TA. The reason for booking a cruise a long ways out is you have plenty of time before a penalty sets in where you could lose $$. As time passes you may find a specific cruise you want (or you may have already had one at the time you booked on board). The worst that happens if no particular cruise comes around that you want, you cancel BEFORE the penalty phase and get your $$ back. Ngaire has taught me well and this has always worked great for us. We always have an onboard booking "in the bank".