Kind of up to you and what you'd like top try.

I've done both.
I just blend with tap to get the KH I want, there's no reason not to really, you use RO to remove some KH, not all of it.

I suppose some might want to use it 100% and then know precisely what is in the water etc as far as KH, but that assumes the RO is 100% removal..............and that's not always the case............

Regards,
Tom Barr