If you leave them down while stationary, water comes back up them and won't do you any favours - hence the bit of string. Once you get going and up on the plane, the water should drain fairly quickly, but due to all that weight at the stern, it may take a wee while for the bow to come down - nothing to worry about
. I tend to put mine down, drain away any water and then tie them up again straight away. Unless you're out in the rough stuff you don't really need them down once you've drained your excess water!