Organic grow? No flushing ever, you're washing away the preciously build up soil - only water for the last 2 weeks.
Personally I add a bunch of vitamins and amino acids and some light molasses as well. Or just feed right up until harvest. Good smell/taste comes from proper drying and curing.
Mineral grow? Not entirely sure about coco but to my understanding, you shouldn't flush coco at all because it is actually counter-productive to oversaturate it, the only reason to flush in coco is if you have an issue with overfeeding or unbalanced nutes then you flush to reset the substrate. You would probably still encounter salt buildup in coco so if that's a concern to you, use just water for the last 2 weeks.
Don't stress so much about flushing, instead spend that time to make sure you have a proper drying setup.