It’s just personal preference.

I learnt it from a chef in Japan in 2009, and I assume he had been doing it for many years at that time.

Generally, that’s something done at a sushi train restaurant where the dishes won’t have wasabi in them already. I’m guessing these notes are for a sushi restaurant where the chef prepares the sushi specifically for each customer, so if you wanted wasabi they’d put it in the sushi itself.

300dpi is the industry standard baseline for printing, 150dpi is not “excellent” it’s barely usable without pixelation being visible in the print.