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.

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!

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.