The main thing is to get it to match up to the port for a seamless transition to the head. This is helpful to eliminate any flow losses from a step at the junction.
Unfortunately, for a carburetor application, the smooth mirror-polish is not helpful, and the normal school of thought is to leave the interior at about a 100 grit finish.This helps to re-atomize fuel that drops out of suspension.
You won't see any modern porting job with a polish like that in it. Even though it looks great, the function is better with a rough interior finish.
Way back in the day, they used to do polishing like that, but it has been changed for quite a while now. Ever since wet -flow testing was discovered, the polishing went out the window.
If you plan to do the heads, please discuss it with me first, and don't touch the port floors please! Porting should not lower the port angle, nor sharpen the radius on the short turn.