under what conditions should the head be shaved whem re-assembling to block? (a15-so alloy head)

is it only if blew head-gasket?
always?
if obviously warped only?

i ask cause im switching block at the moment, and wondering if i should have it done, i dont want to do it as it will drop the valves and raise compression (not good for turbo) oh yeah, and it costs money...

Run a straight edge over the head & see how much it is out of true with a feeler guage.
My Sunny [B310] manual says that the maximum thickness feeler guage that you can slide under the straight edge must be less than .004" or 0.1mm. If the out-of-true is less than these figures, you can bolt the head back on.
This is for a stock engine. For a boosted engine, i would personally cut that measurement in half.

Lay the straight edge diagonaly along the length of the head [both diagonals as well as straight along] & check it in many places, but between the combustion chambers is always a good place to measure

Always check the head for true whenever it is off. It's quick, easy & cheap to do & it could save you needing to take it off again if problems develop.

cheers

I can give you the name of a good guy in Springvale if your going to go ahead with it.

Cheap too, he charged me $125 for head and block shaved, hone and all hot tanked.

I thaught that was pretty good.