Normally the standard options would be used as described below:
Open - When entering a brand new potential lead.
Qualified - Lead meets a set of criteria based on your company's standards and/or is interested in becoming a customer.
Unqualified - Lead does not meet your criteria and/or is not interested in becoming a customer.
Yes you can use the Lead is Active? checkbox in the EditLead screen and if you're already using a customized LeadList screen then you can easily add this as a column or use it to filter, etc. Additionally, if you want to use the Open or Closed option you can in the Opportunities screen, unless you're currently using it for your own purposes.
- Ashur