Depending on the window and how it being installed.

Here is a few examples of flush and recessed installation.

For flush installation withwood bucks, would it make sense to use that flex wrap and weather flashing tape?

These windows are egress basement with no siding around.

If the windows are retrofit into block or masonry, there is no application of flexible flashing tape in that instance. You can use a buck frame (use PVC) and they should be capped and flashed with coil at that point, but it is not like an install inside of siding that requires flashing behind the primary cladding.

This is new construction. Here is what the openings look like.