Pi-hole works great on it’s own to block ads for most people. However, you may still see benefits of using a browser-based ad blocker. Many of our users use Pi-hole + uBlock and they work very well together.
Pi-hole doesn’t remove the empty space from ads, so you may still see big empty areas on Webpages. Most browser ad-blockers can help clean this up, which is a solid use-case for running both.
Running Pi-hole plus a browser extension also gives you a layered approach to blocking: Pi-hole will stop what it can, and if something gets through your browser extension might pick it up.
An argument against using both is that Pi-hole only needs to be installed one place, whereas browser extensions need to be installed to each device.
In the end, it really depends on your preference; there’s no right or wrong answer. Experiment with different combinations and see what works best for you.