from the B21/B23 reconditioning manual.

You can grind the seats. You can grind the intakes.

BUT, you lap the exhausts!!!

...I lap the intakes onto the seats as well...

...and I go for middle to mid-high seat width after lapping...

[it's a heat transference thing.
I WANT my valves to be able to shed heat into the head. And through the seat to the head is a VERY good way to shed that heat.]

Another one of the little things that can make the difference between life or death in the environment of boost heat loads.