Force users to select a task billing rate by setting the field as required by default. 

This will enable you to accurately report on where your team is spending most of their time on and flag problem areas in order to increase revenue.

Follow these steps

1.Hover your mouse cursor over your name.

2. Click on Company Settings.

3. Open the Opportunity / Job Section.

4. Toggle the setting that says "Task Billing Rate Required".

5. Click on Save.

5. When adding a task the field will show as required*.

Also see: 

Forcing users to track time

Setting up your billing rates / item types

Next Step:

Pull a User Profitability Report at the end of every month