I asked my stylist Di Di, (NYC hairstylist) that same
question yesterday ...

She replied: If I wanted to shower with the piece on
... wash my 'real' hair on the sides ... and only use
'color-safe' conditioner on the piece .. while attached
.. always making sure I kept my head tilted back
under the current of the shower spray ..

so the conditioner and water would flow down
from the hair down my back ... and not into the lace ....

I haven't tried her suggestion yet ... but will this week
after a workout at the gym I'll give it a go.