IMO, not required.Should I preemptively install OC now?
1) Catalina won't do anything better for video editing with 7970. No point to make the setup more complicated with no benefit.
2) 7970 cannot benefit from OpenCore to get HWAccel.
3) 7970 can be flashed to get native boot screen, don't need OpenCore boot screen.
But if you go for anything between RX460 and Radeon VII, then yes, you better install OpenCore, and get full HWAccel in 10.14.6.
Of course, there if nothing wrong to install it, learn it, and see what it can do (e.g. allow your cMP to install and update Catalina / Big Sur natively). Especially you are looking for a video editing build. Upgrade the GPU allow to use HWAccel should be much better (unless your whole workflow never need to deal with any H264 / HEVC videos).