I used to tell people that yes, it is better to put together your manual talent review-succession strategy, tools and processes, and then find a talent management system to support that.
But now, I advise people to start looking at talent management systems early on, especially if they are a mid-large sized organization with thousands of employees, because:
1. It takes a long time to research systems, to view demos, to complete an RFP process, to purchase a system, to train everyone on the system, to merge and enter data into the system, and to really make the system useful. So you don't want to wait until you are tearing your hair out trying to keep up with all of your manual talent-succession tools before you start looking at systems.
2. If you decide to use certain tools in your talent-succession strategy, such as a 9-Box, you want to look at systems to see how they portray this tool, so you don't have to pay for system customization, or try to retrain everyone on the "new system 9-Box" when they have been used to using a different version of the 9-Box tool.
However, it is still important not to let a talent management system dictate your talent benchstrength strategy, tools and processes, which should all be tied specifically to your business needs, to your culture, etc. Getting talent data into a system is not the goal - the system is a tool - not the strategy and certainly not the end result.