According to instructors, a good Spey caster learns all casts from all sides. I was off to a good start when I lived out there by paying attention to the left side as well as the right side. Funny how when I go back there I see how much more advanced everybody gets during the year that lapses between visits. (the majority of my two-handed casting during that year ends up being overhand casting in saltwater). The PNW guys on this board include some very proficient Spey casters. In fact some of the best on the river.
My logic behind a decision to single Spey cast or opposite hand double Spey, etc is driven by the surrounding factors. Wind - upriver or down? Presentation - cross stream or down and across? Clearance - tight to the bank or plenty of room? Distance - screw the wind and go with my best cast, or even overhand it with a false cast.
When I fish alone, I like to work on the casts that I am least proficient at to try to bring them up to speed. Someday I hope to have no "best cast" and no "insufficient" Spey casts, so that just the conditions control what I do from both sides. Not there yet!