While it gives the modelers a nice reference, it is not a standard practice. Usually only very large studios, with even larger budgets do this, as then they can employ a full time sculptor. For example: Pixar has an artist named Jerome Ranft, Joe Ranft's brother, who is the full time sculptor there.

However, doing this also takes a lot of time, as then the sculpture needs to be scanned. A very long, and laborious process.

I've had to scan multiple things at work using our scanner, and I can say it's very difficult to get good results. Lots of clean up, fusing scans, and then having to re-topologize all the data. The captured textures (if there are any) also need to be redone.

So no. It isn't required.