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ETA: sometimes I have enough points for just one-way on one ticket, so I'm essentially using points to pay for 2 and a half people to fly and then cash for the other one-way. I just wanted to make sure you didn't overlook that option. It's nice to use points as much as possible.
Yes, that is absolutely crucial to know. Due to capacity controls, purchasing flights with RR points is not always possible on certain routes on certain days (for instance, try booking something from a coast BACK to the Midwest on a Sunday after lunch -- it almost never works). However, it may well be possible to book the entire party in one direction on points, and then pay for the return, or vice-verso.
As a general rule, if there is capacity, we try to book the flights on the more expensive days (ex: Fri, Sun, any kind of event-travel day) on points, and pay cash in the direction that is cheaper.
Also, since the OP has a non-family member in her party, let me mention another issue that it pays to be aware of: if you have any person in your group who is outside your control in terms of schedule (a child's friend, for instance), or anyone of any age who has demonstrated a tendency to back out of trips in the past, it is best to book that person's entire trip on points wherever possible, EVEN IF YOU HAVE TO BUY POINTS TO DO IT. The reason for this, as the PP said, is that credit for
cash fares purchased but not used can only be held for use at a later date by that same passenger, but that credit for unused fares purchased on
points will pour back into the account of the person who originally purchased them, and can then be re-used by any person who the originally purchaser wants to use them for.