or just the ones that will be different than the variable name?

that's a very good question. for some reason whenever I see sample code for kotlin, the column info annotation is always added... maybe it has something to do with how it compiles to java?

I don't think its specific to Kotlin. I use

@ColumnInfo

if I want to set a different name for that variable from that provided as the parameter name

If you use proguard, it will obfuscate your variable names. I made that mistake in the past, not explicitly assigning names for json serialization, never again.

Room processing is different: you are not forced to use

@ColumnInfo

in order to use proguard and obfuscate your entities; there is no reflexion mechanism.