"If not porting, how should it be done when installing bigger valves?"

IMO, if you're planning on installing larger valves in an effort to increase the airflow capabilities of the heads, then some amount of porting should be considered as a mandatory part of the big valve upgrade.
In fact, if I had to chose between mild bowl porting or bigger valves, I'd take the bowl porting, especially if the heads in question are 346/902/452 heads.