A key to your "problem" or "fortune" would be your specific altitude in Boulder, CO. See my chart on this page for an interesting read: How does high altitude affect brewing?

For the Rocket line of machines in your particular case, I would say that it is best to flush a few ounces and brew, i.e. you would be following a flush-n-go procedure. This is, by a goodly margin, the most common way of preparing a drink. Backflushing has its place but not here.

Typically I have seen and read that Rocket machines arrive at the doorstep with a maximum boiler pressure reading of 1.10 bar. For the average user, a flush of 5-6 ounces and brew would be fine. But for you in Boulder, CO, a lesser flush would be better because you are starting off at a lower temp due to the altitude.

Has Matthew's idea been dismissed prematurely? I know nothing of the Rocket design. That said, I believe Quickmill locates the overpressure valve after the heat exchanger. So, I would expect backflushing a QM to flush the superheated water from the HX back into the tank, rather than out the group and into the drip tray. Observing the end of the water dance might become a little trickier. But you HXers like a challenge, right?

Disclaimer: I don't own a HX, and am probably highly unqualified to participate in this thread. However, having to waste 5-6 oz of water before each shot on a non-plumbed machine was a big factor in my choice of machines.

If you brew 6 doubles a day, and flush 4 ounces per pull (you won't), that's only 24 oz of water "wasted," about the right amount of ice to make two martinis. You waste substantially more in the shower or on the john. There are better places to save water.

It's hard to say the flush is wasted though, when it's so important to the process. It serves two major functions.

Important but secondary is stabilizing the path by harmonizing its various parts; and water which doesn't flow through the entire path, including the group and pf, can't do that. This matters most after the machine has been left idling for any length of time.

More important to the practical art of brewing with an HX is learning to read the flush accurately, to know when the temp is Goldilocks' "just right." Backflushing is helpful but doesn't convey enough accurate, real time information.

Machines with quick rebound should be flushed before every shot.

BDL

Point taken about cooling the group. However, my point (and I believe the OP's) about wasting water wasn't motived by some hippie "save the environment" mentality. My tap water is too hard to brew with, so I rely on bottled. More water used means hauling more gallon-sized jugs home from the store, more futzing with the tank under low cabinets, and more chances of forgetting to empty the drip tray. Thus, water "efficiency" and convenience are closely related.

Oh well. It was worth exploring.