What/When color gels do you use? And why? (vs shifting your WB)

Yes. And the camera is set to 3500K°. I never use auto wb. Nikon speedlights are closer to 6500K° than true daylight (direct sun), so they need even more orange than expected, if you want to match domestic lighting, wich is closer to 2900K° . Another reason why some gels will not look  perfectly balanced, (they're all made to be used with incandescent light, neon or daylight) but they may get close enough for some uses...

When you set the camera to 3500K°, the ambient incandescent  light will be balanced, but the light from the speedlight if not gelled will be way more blue. It usually looks ugly.  That's the idea: match both ambient light and light from the strobe, so that everything is in the same light colorwise.

That is if your settings and desire is to include some of the ambient light. If you're at ISO 100 1/250 and f:8, the ambient light of interiors will probably not even record, so there is little advantage in gelling in this case.

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Jean Bernier All photographs are only more or less credible illusions