"Assuming that their time machines can displace them spatially and not just temporally"
There's nothing in the movie to suggest they have precise control over where they end up.
Grace appeared in the middle of a highway and the time machine ball burned a hole in the highway which she fell through. That implies (to me) that they missed their target by a couple feet, or they didn't know the highway was there.
Likewise, Kyle Reese also fell a few feet in previous Terminator movies: Terminator and Terminator Genysis.
So if you're going to send someone back in time but can't precisely control where they land, it's safer to send them a couple feet higher so they don't appear underground. Otherwise they could be entombed and quickly suffocate or get buried.
Terminators have the same danger of appearing underground, and they're capable of
falling from greater distances so why not overshoot as high as possible?