Only time you would mess about with the spark plugs is if you are seeing pre-ignition (usually because the engine is heavily tuned) in which case you would go one stage cooler. But I imagine this is very rare to see in the N20 engine.
Bit of reading for the N54 engine (35i/35iS) that may shine some light: https://www.bimmerworld.com/About-Us/BMW-N54-N55-Spark-Plugs/
