We are going with a freestanding in our new master bath both because we like the looks and also with our layout it worked better. Given the cost of a good tiling job I'm not sure a built-in would have been less expensive than a standalone. We had 2 drop-in tubs (one to be removed with shower stall) and one of them still in-- acrylic-- which we put in 12 years ago and somewhat regret now because it flexes and the grout cracked over time so it has needed a lot of maintenance. (this is mostly because of the material we chose). But when our kids were younger they loved the bubbles!

Install a large operable window in the bathroom that extends to the floor and place the faucet side of the tub up against it; that way the window can be opened and faucet serviced from the outside. Things dropped beside the tub can be retrieved through the window also.

Or does the tub filler get secured onto a stringer installed between the floor joists?

Apparently.