Hi,
First of all, this question shoudn't rise in any organization if the employees are having enough project work and task to work. Its responsbility of the management to ensure optimal utilization of the employees. Also management MUST encourage employees to put the ACTUAL working hours in their timesheet. This is the KEY metrics for Managers to do load balancing and also to forecast staffing for future projects.
Exceptionally, if an employee is on bench (internally within an organization), he must be allowed to put his timesheet as 'Available for assignment' with 40 hours (5*8).
Per my knowledge, there are some outsourcing companies which forces employees to put > 40 hours a week (for billing the client) irrespective of whether they have performed any working or not and this is an another exception to above concept. Hope this helps.