If you do it while attached, you`ll be applying shampoo neat onto the hair....I do it in the sink in order to [a] sit the system into diluted shampoo and [b] to get a good 10-15 minute `conditioning`--you can`t really stand in the shower that long while conditioner `works` and [c] there is a risk of hair getting tangled etc. if doing those treatment in the shower under a jet of water.... I prefer the sink method as I can `nurse` the system.... it`s up to yourself really, but product can lodge around the bonded part under the lace, and may not be fully rinsed off in the shower...can lead to issues like irritation/bond weakening etc..... just my way ...other guys do their cleaning in the shower ... ..I used to when I had all-poly systems but I got tangling issues, so changed to the sink routine with lace systems. Cheers.