I think it depends on the specific tankless heater. I believe the newer models from the higher-end brands have gotten more sensitive in terms of turning on at lower flows. I'm thinking Rinnai, Noritz, and Takagi as examples.

That said, the Watersense spec is for flow to be in the range of 0.8 gpm to 1.6 gpm. I think that should turn on any of the above.

We have an older Noritz, and IIRC it turns on somewhere around 0.8 gpm to 1.0 gpm.

It did require a little adjustment- for example, my wintertime shaving routine changed. Rather than try to trickle out hot water to rinse the razor while I shave, I close the drain and put a few inches of hot water in the basin. Then I just run the faucet full-open until I get hot water.

FWIW, if you're re-doing piping, it may make sense to go with smaller branches to each fixture instead of one big header with short, smaller risers. Will help somewhat with the time to get hot water. And I'm thinking about hard piping- PEX with it's manifolds and home runs is more amenable to this as a default.

Here is a link that might be useful: Watersense faucet specs