Different people do things a different way, but no way would I connect into a coil feed, cossies suffer enough misfires as it is due to poor parts/design, although convenient to switch from here there is no need, personally I would just have the relay wired with the fan switch with a permanent feed so it can run on after shut down, if this tiny amount of draw makes any difference to the car starting etc, then you have other major issues anyway, but as I say everyone is different.
Like you say the fans are pretty much on constant in traffic on a hot day, once on they certainly don't like to go off, I found exactly the same when I fitted the deflector plate, which runs from behind the bumper to the gap between the rad and intercooler on a std cossie, I didn't have one when I got my car and like things to look std/ish to a point, so fitted one.
As soon as the hot weather hit and it took 5-8miles of slow 50mph driving to put fans off,(as shown by my led) I removed it and junked it, wow what a difference it made, although now drawing heat into intercooler in traffic I reasoned no harm as it won't be doing any significant work in traffic anyway, and once on the move the flow of air through grille will cool as normal the intercooler.
As you say above your fans are going off you don't need to change anything if you don't want to, it's just I would not connect anything on the coil personally or have it ignition switched, as the water gets hotter after shut down due to absorbing heat from the engine, and having no rad air or water flow.
tabetha