In Defiance, Upheaval deals more damage than IB (damage increases with current HP & HP increasing effects i.e. the +20% from Defiance & Thrill of Battle) so if you're not bothering with stance swapping (or you don't need the gauge for IB), use it whenever it's ready.

Stance dancing when you're main tanking is still a thing if you do it right. With Inner Release & Infuriate, you can switch from Defiance to Deliverance, Fell Cleave x4 then switch back to Defiance. If you don't have those cooldowns ready, dump your gauge then switch OR just switch and deal with the loss of a few stacks. If you line up a bad swap with Maim + Storms path, you'll fill back most of what you drained in 2 global cooldowns.