You can do whatever you want but I almost always set it in Pro first personally. Just make sure to check that the symbology is compatible will all clients, otherwise it might not show correctly in all apps. I just feel like modifying symbology is easier for me in Pro.
Thank you @Laura , I appreciate your input. I know I can take which ever approach I want, but I was wondering which would be the best approach, particularly when it comes to performance. However, I am beginning to think it doesn't really matter as any symbology created and published from Pro is replicated in Portal > Web Maps. So either you publish with symbology or create symbology in Portal Web Maps, either way, the symbology will exist in Portal.
Thanks again!