Rule 7 mentions that for assessing if there is risk of collision or not, compass bearing of the other vessel should be taken. If the compass bearing does not appreciably change, there is a risk of collision. 

It does not mention true compass bearing or relative compass bearing. Actually in most of the cases it does not really matter if we take true of relative compass bearing of the approaching vessel.

There is rare possibilities where even when relative bearing of the approaching vessel changes but the still the risk of collision would exist.

Read about this sitaution by clicking here.

And when we have to take true bearing of the approaching vessel, we can get it straight away from the gyro repeater or from radar.