I’ve almost always built opposing sides. I can only think of two occasions, where I did not, and in both instances, my opponent moved out of state not too long after building the army. The result was old GW Imperian Guard that went 17 years without seeing a tabletop (eventually re-purposed for other sci-fi and post-apoc), and 28mm Celts who are nearing the same.
For most of my first 20 years in miniatures gaming, I usually had an opponent, but I still expected to play solo games, so bought both sides. Over the last 20 years, I’ve moved to predominately solo gaming, so building both is a must.