Sometimes. Port flow on an engine is VERY tricky. Most high-po manifolds are designed to equally ditribute the fuel/air charge so you want to keep head port flow equal. But on different engines with diferent combos the port flow may not be equal and will need to be adjusted somehow. But this is not usually done with head porting. Head flow is best kept consistent. Adjustments can be made by changing rocker ratios on the corners. It sometimes helps you pick up a few ponies if you run 1.5 rockers on the inside holes and 1.6's on the corners in a SB Chev. But that method might not work on other engines. This is one of those things where you ask 5 people and you'll get 7 answers. My answer:for most people, don't worry about it. Just use good components and don't "over modify" unless you know what you're doing and you'll always build a strong motor. Any other Q's, don't be afraid to ask.