Working with Calculate and Approve in Time and Attendance , it should be fine to have the same inhereditance rule for PROJECT CATEGORY when inserting new line by PROJECT or PROJECT ACTIVITY for actual/planned work made.

The rule  should be the same when inserting line in a PROJECT HOUR JOURNAL (category is inheredited from Employe, i.e. in the employee project setup, or from ACTIVITY PROJECT, i.e. Work Breakdown Structure and so on).