I have 2DC aged 17 & 18. They are both on our apple family plan and I can track them via findmyiphone. They both know about this and are happy for me to do so, when necessary.
When my eldest turned 18 & started going out clubbing, he kept forgetting to text me to tell me when he was home (circa 4.30am) and I kept waking him up accidentally by checking if he was in his room (Ive got a Samsung now and its a faff to use findmyihone as I have to use my Mac)
He actually downloaded snapchat onto my phone so I can check his location on the map as he has shared it with me and his sister so it doesnt matter if he forgets to text me.
I rarely check it but its good to know its there in case I do need it.
We are a very open family and very little out of bounds for them. We talk a lot and know their values are similar to ours in as much as they can be when you're a teenager. I lied to my parents ALOT when I was their age about where I was as they were overly strict and restricted where I could go and with who. Because of that, I am a very different parent to mine and we've never had any issues with lying about where they are or what they were doing.
Ive been known to collect them and their mates from clubs and pubs before if its got too late and the wait for a taxi is too long. Being woken up at 2am to get them isnt unheard of.
I dont track them as a matter of course, I only ever look at it to check if they are home after a night out for their safety and I dont usually need to do this anyway as i often get woken up as they get out of the taxi, or Im up with younger children in the night.
I think its got its place from a safety point of view, but it should be completely open with everyone knowing the ability to locate them is there and not used as a mean to spy on them.
They've never had reason to lie to us about where they have been as we are reasonable parents and also not stupid. Weve allowed alcohol since they were 16 as they were going to do it anyway, nights spent camping at the beach with mates etc so there hasnt been a need to lie.
However, due to my different relationship growing up with my parents I would have gone apeshit if they had been able to do this to me, but I dont parent like they do and I trust my kids and they trust me which I suppose is the difference.