I say practice standing, cause if you start playing Live. More than Likely your gonna be standing. Practice both Distorted and Clean, you will use both later on. If you are doing any finger exercises and stuff like that. I say do it clean, cause when your distorted it's harder to pick up on if you accidently played a note half-muted or bumped the wrong string slightly. When your guitar is distorted, even alittle sloppiness is covered up, when Clean your ear is more discriminatory to your playing. When just jamming around, It's more fun to be distorted (for me anyways). That's the best reason to play, cause it's fun.