MRC most of the time while playing it you are looking at the notes you're playing. It's like reading a language.
People don’t read word-by-word, they read by phrase (which is exactly what you said about playing).
The other thing that we’re not really articulating is that, regardless of wherever we are activity “looking,” we see a lot more than just those notes. And that I think is key, because we’re taking in a lot of information. And then longer you’ve been playing (and the more comfortable you are with written musical notation) the more automated your process of interpreting that information becomes.
So maybe the challenge of sightreading (one of the challenges anyway) is figuring out which parts aren’t yet sufficiently automated, and making sure you’re making up for that in how you read.
So back to “start at bass clef vs treble clef,” perhaps what matters is for the person to start at their weakest point. And this might even be piece-specific. Or maybe even changing within one piece.
What I mean is, if the LH part is pretty straightforward, I will need less time to translate what I see into what I play for the LH. In that case, focusing on the treble clef and sort of glancing through the bass clef as I play, may be sufficient. But with a more challenging or complex LH part, I may need to be actively reading that more, then reading from the bass clef up might be better. Especially if the melody is clear and it’s easier for me to predict where the melody is going.
In any case, I have to go to work now, see y’all later today 😅