Generally, customizations should be done starting with the port.
- The hard work of determining dependencies and OpenBSD-specific provisioning has already been done for you. This makes it far easier for you to begin any patches of your own, or provisioning changes to how the port builds.
- Your local customization may be useful to other OpenBSD users. If you wish to, you may contribute your customizations back to the port's $MAINTAINER as a $FLAVOR, or as an enhancement to the port.
Edited to add:
Best practice is to copy ports you are modifying from the tree into
/usr/ports/mystuff.... which keeps your own developments separate from production branches. This same branch can be used for new port development.