With the prices of the RTW tickets going up on a regular basis it is always a good idea to book as early as possible. If you don't need the ticket later you can return it for a reasonable fee.
Then you run into the problem that you cannot book flights more than 11 - 12 month out. The ticket itself is valid for 12 month after the first flight. You can buy your ticket almost a year before the first flight, locking the price in.

In that case you book the first flight with a fixed date / flight number. Everything else can be open. Sometimes you need to book more than the first flight, depending on how you start.

If you keep the routing you just make reservations for the flights. If you change the routing you reissue the ticket, also a reasonable fee for this. Do all the routing changes in one transaction.

The risk you have with open segments: if you have an open segment AAA-BBB and there is no more flight you have to reissue the ticket. If you are already at the 16 sector limit and need an extra sector for doing this you have a problem. Same on Star when you are at the sector limit or at the mileage limit.