Topping Resivoir During Flushing?

Okay, so I have been flushing for a couple of days and normally, it would be time to top off my res. Do I still top it or should I let the water level go down a little? Thanks for the help!

Come on guys, it may seem like a stupid question, but can I at least get a stupid answer? I am asking because, during my research and reading post here on RIU I have heard that soil growers let the soil and plants dry out a bit before harvest. I am just looking to find out if hydro growers do something similar. It's my first grow guys, have mercy?!

EDIT: I just realized, some people might think I am talking about adding nutes. I'm not dumb, I am talking about adding pure water.