Sib, with an auto, you might not have to coast, depending on the transmission programming.
I don't have a Subie; I have a 16-year-old full-size Chevy pick-up with a conventional four-speed auto and my wife has a 10-year-old Ford Focus with a four-speed auto. Both vehicles are equipped with tachometers.
The pick-up seems to be a little smarter, at least when I'm using cruise control, despite its greater age. When descending with cruise, it is capable of generating more engine braking than if I don't use cruise and simply take my foot off the gas. When it needs less braking, it seems to unlock the torque converter before it adds RPMs. On some grades, that's enough to allow it to maintain the desired speed and the engine continues to tick over at low RPMs (but more than idle, which is only about 500 RPM on the V8). Mind you, it doesn't work the same as a manual, but it works remarkably well.
My wife's car doesn't get much engine braking, especially in 4th, whether or not it's on cruise. So it "coasts" just fine even while in gear, but more brakes may be required.
Unlike a manual, an automatic is capable of allowing plenty of slippage and low engine RPM while descending, even while in gear. The RPMs may be higher than idle, but usually not very much, at least in the cases of the two vehicles we own.
Extreme examples of modern automatics can be found in full-size pick-up trucks. The cruise works with the engine computer to manage fuel flow, torque converter lock-unlock and transmission gear selection. For example, they are capable of downshifting on descent in order to achieve more engine braking and upshifting again when less (or no) braking is required.
I drove a full-size Chevy Blazer for years that had a four-speed manual with an unsynchrornized granny gear. I also had a Triumph TR6 for quite a while. I'm not completely ignorant of manual transmissions. They're fun, but modern automagics work pretty well.