We (me, dh and 17y dd) all have Find My iPhone on our phones so could do at any time. We don't do this randomly though.
We do sometimes use it though but the other people know if and when we will do so. For example, I use it on certain evenings when dh and/or dd are heading back and I am getting a meal sorted. Rather than them having to phone we will use it to give us a good idea of when we will arrive back.
If meeting dd to collect from a school trip, or if were picking each other up from the station, etc. we use it to check they're nearly there, etc.
DD's used it to figure out where me and her dad was, and used it in conjunction with directions/map to meet us.
There are times when you know the person won't be able to call easily but it'd be far handier to know if they are nearly back or on their way, etc to make plans accordingly.
Find not to, but just having the ability to do so, and use it occasionally when we are all aware of it happening works for us. There's no checking up or snooping going on - well afaik anyway. I don't. I trust dh not to, and I really can't imagine dd doing so.