Use it when you've got explicit permission, not just because there's no rule against it.

Permission Nuance

It suggests a choice, meaning you're not obliged or expected to do something.

Not Obliged

It's more formal and may sound outdated in casual conversations. Use it in writing or formal speech.

Focuses on personal decision-making power without external control or influence.

Can imply a suggestion or encouragement to take action, not only the possibility to do so.

Being 'at liberty to' can indicate that someone has trust in your judgement or discretion.