I'd wait as long as possible (especially if it involves asymmetry), but before making hair. When you get to the Multires Sculpting part, you'll see that you can sculpt on the base geometry, or make changes in Edit Mode to that base, and all the details, like pores and wrinkles will 'follow' the changed geometry.
Now, I didn't do this at the beginning of the Course, but at a certain point, I decided to watch the whole course untill the end and then continued where I left off. It is long, but it is definitely helpful (and entertaining!). It is also what Kent recommends, by the way. And it will become more clear, when to add an expression (there is no strict rule though!).
Spikey's advice is spot on. With any course, I recommend watching once all the way through before implementing unique changes. UNLESS you're already skilled in the course topic and can learn from it more flexibly.
In this case, you could add an expression at the end or in the middle. Both have their pros and cons:
If you implement an expression in the middle of the course, you risk losing the utility of symmetry for a variety of tasks later. You could add the expression as a shapekey but you'll also have to add shapekeys (or keyframes) to the objects that are affected by your facial expression. For example glasses that would move if the head is tilted at all. It's very doable but adds a layer of complexity in my opinion. If this is your first portrait then it may add unnecessary complication to an already complex workflow.
If you add an expression at the end, similarly you'll have to adjust all objects that are affected by the facial expression. Notably all the hair emitters. I'd recommend this approach because it doesn't complicate the workflow from within but rather after the fact.
Creating a simple face rig might be the cleanest way to pose at the end. But rigging is another beast by itself.
Thank you for the thorough responses! I'm thinking I'll save it til the end (possibly before hair). This is my first portrait, so I'll take the not-too-complex-in-the-throws-of-it approach lol. And I've been reading a reasons to make myself learn more about rigging anyway 😅