Hi,
First of all, this question shouldn't arise in any organization if the employees have enough project work and tasks to do. It's the responsibility of the management to ensure optimal utilization of the employees. Additionally, management must encourage employees to input the actual working hours in their timesheets. This is the key metric for managers to balance workloads and forecast staffing for future projects.
Exceptionally, if an employee is on the bench (internally within an organization), they must be allowed to mark their timesheet as 'Available for assignment' with 40 hours (5*8).
To my knowledge, there are some outsourcing companies that force employees to report more than 40 hours a week (for billing the client) regardless of whether they have worked or not. This is another exception to the concept mentioned above. Hope this helps.