Pairing is optional at my employer.

As a manager, I'm always on the look-out for tasks where I feel pairing may provide value. These are often moderately complicated tasks where I want a more junior team-member involved. Or, doing green-field UI work, I'll suggest one of the more UX-savvy developers pair with a small group of developers.

I have also managed people who just like to work together and they're free to pair and unpair on an ad-hoc basis. I don't care, as long as progress is being made.

Edit - I've never "paired" in the way you suggest in a sibling response (assigning a ticket to 2 developers).