It depends on if your rail cues are a vista or during scene shifts. In my experience, when I'm calling a scene shift and we have the opportunity to hide the shift (we bring in a DS Border and do a small scene in front while everything upstage changes) I start the call of the shift, then the Head Carpenter takes over, as he can see everything going on upstage while I focus on the scene downstage. My ASM is more mobile than I am, so she also keeps an eye on things above heads to make sure nothing is about to hit someone or something as it comes in.
Any time a rail is flying in, someone should be watching it - I work with IATSE crews, so we do have 2 head carpenters are either side of the stage that watch as rails come in. They make sure that the drops don't catch any air, keep an eye on the rovers, dancer heads, etc.
My most intense and dangerous rail cues were a vista for the ballets: Romeo & Juliet and A Midsummer Night's Dream. For Midsummer, my Puck would run under rails as they were speeding towards the deck, to give the idea of traveling - and once he reached his destination and turned around, we flew out the rails in the reverse order so that he would arrive back to where he started. That sequence was very frightening for me, but the dancer put his faith in the crew, was the same every time with his running pattern, and everyone kept eyes on the drops as they came in. The rail crew also has a video monitor with them, so they can see how it's coming in from the FOH perspective. As you mention, the "placement" of the cues - I called the cues based on the music and then the choreography of Puck. Should he be behind the music, I needed to wait until he was in the right spot on stage before I started calling the rails - otherwise we would get a little too close for comfort (should a rail get too close, a hold is called, or asked to slow down, if possible). It's also important to note the desired speed of the rail cues when they are a vista. For the scene mentioned above, they had to be fast - as it was Puck's running sequence. For a scene that involved a giant spider web covering Titania's home at a scene break, that needed to be medium speed - to match Puck's gesture to bring it in. During tech we rehearse and find the desired speed, and if the request is "as soon as Puck's hands reach up to call in the spider web, we want to see it" then you need to find where in your music/blocking/choreography is the best time to call the cue, so that it does appear when requested.
Please let me know if this was helpful - if not, can you rephrase your question?