I don't want to speak for David Finch, but here is a pretty popular example of Neal Adams using tangents to create a face of Deadman. I think it's important to understand rules are not there as hard never break things, they are there to help people self-correct their own work. In this case how do you avoid making your page look flat. As far as your page, yes and no. The tangent doesn't bother the flow but the first panel doesn't establish a solid environment so it looks a bit like a floating object with no dimension. The second panel I understand it's zooming into the mouth but it might be framed too tightly for others to read instantly. Most honest feedback I can give.

Apart from looking at how others do it and what you like there: Draw a full head to shoulders, cut out the panel shape (or many shapes) from a different sheet and play around. Or you take a head photo shot and play around with crops in photoshop. That should give you an idea of what you like. It will probably depend on the focal point of your storytelling (creased forehead, shocked eyes, wrinkled nose, clenched teeth, all of those) And when it comes to panel arrangements: Check, if any line, that gets cut off by a panel border, is picked up at the next panels border. If so, it might connect the images in an unwanted way. That problem is not exclusive to head and body crops, it can happen with any line.