Green Card Reimbursement - what are the best practices?

In regards to Green Card Fees, what are the best practices for the following:

1. Your company bearing all the costs associated with the process (fees, labor cert)
2. If you do cover the costs, what or if any reimbursement do you require the employee to cover if they leave after a short period of time. (please include your definition of a short period of time and what costs are open for reimbursement)

Thanks,