I've a decent RO system that I've started using for my coffee. What I notice is that if you use pure 0 ppm water (distilled or RO), minerals will come from somewhere in an espresso machine, in this case likely your boiler, or scale that has previously built up inside the boiler. Or in the case of HX machines, less so but some taste will come from the inside of the HX. If you do use pure 0 ppm water with no time for minerals to leech into it, I feel extraction is thinner by taste.
I would suggest to add back a bit of tap water, or better, to add something like a piece of bamboo charcoal (you can usually find in places that sell Chinese tea, or online). Gives the water some minerals and a nice taste IMO. I toss a piece into my machine's water reservoir.
Most of this is just anecdotal and based on taste, I don't have a TDS meter, but you can do a function test. Run some water through your machine and taste what comes out, is it good? Metallic? Rancid coffee oil? Will tell you if your water is good, and if your machine is clean.