As has been mentioned, placing a 'full' block then chiselling to shape seems most immersive... (for a similar example see the game 'Vintage Story').
So, place block => select chisel tool/shape => Hit a few times
Maybe tool should retain last required shape?
Since often when building a block is getting placed in position within a few millimetres or even partially inside another block i don't see that as a good option.
Chiselling before placing is fine. Just not.. You know..