Taking the recent screwup out of account... It's tough. A commercial product shouldn't try to assossiate with another brand. But if we're being realistic: "Her" is nearly uncopyrightable.
Since Her isn't a tech branch it would be hard to get in trouble based on that association alone for some future company. Kind of like how Skynet in theory could have been taken by a legitimate tech company and how the Terminator IP owner would struggle to seek reparations (to satisfy curiosity, Skynet is a US govt. Program. So that's already taken care of).
As long as you don't leave a trail, you can probably get away with copying Stewart. But if you start making Star trek references (even if you never contacted Stewart), you're stepping in hit water.