I always thought it was odd that he didn't correct himself since he was technically king because his father was dead
1. Training with King Kai, he first uses the Spirit Bomb.
2. Goku formed the Spirit Bomb vs Vegeta. Doesn't count, as Krillin was the one who actually used it.
3. Hit Frieza, but didn't kill him.
4. Killed Kid Buu with it.
5. In Tree of Might, he killed Turles.
6. In Super Android 13, he absorbed it, technically using it.
7. In Dragon Ball GT, the Spirit Bomb was used against Omega Shenron.