Some clients specifically indicate in their stress specification not to use the allowable liberal stress. I have an experience with one of our clients.

This will increase the system's reliability, especially if the line is operating at a creep range.

Just make sure to check the client specification if it's written there. If not, the code I believe allows using this, since there is enough margin of safety for allowable stress, considering other factors such as SIF's.

Cheers!