There's a huge "it depends".
If I've worked on a customer's environment, I usually have a better feel for its requirements than any automated tool from any vendor. I know I've run the VMware Capacity Planner before and had it churn out a recommendation for a total 1TB of storage.
We sold that customer 6TB of storage because we knew they would grow to fill it in twelve months, and that's exactly what they did.
I would hate for anyone to claim we "oversold" based on the uneducated output of an automated tool.