Low-tech solution in my driveway: Put the car into the desired position, then hang up a couple of old tennis balls on strings to just touch it.

Do hanging tennis balls (and their expensive gadget relatives, laser pointers) help with backing in straight?

In my mental model, the tennis ball tells you when you've backed in / pulled in the appropriate distance. I'm not seeing how it would help me back in straight / centered in my garage, or at least not until I've already backed in so far that I'd need to pull out again to try again.

Usually "good enough". Garages aren't big enough to be completely crazy in angles. People pick a consistent location for the tennis ball. Their face, a sticker, a mirror, etc. If they are off, its not going to line up correctly.

My fire hall has spray-paint lines to align the rear wheels with in the mirror, including a short cross-line at the stopping point when backing in. Works great!