This has been asked many times before on this site
I'm sure they are posted away in the archive.
Its no surprise that almost all the posts are of those that religiously close their seacocks and loudly proclaim such. Ross being the brave contrarian.
I will admit to not always closing my seacocks, but regularly checking my boat and always keeping a close eye on things (and everything is double clampled.) If I was gone for a long time, I would most likely close them. I know my boat pretty well, and just because i don't close my seacocks doesn't mean I don't inspect and know whats happening with hoses and such on my boat When there is any anomoly, how ever small, whether a sound or whatever, I never let it go. I will investigate until I know whats going on with ever inch of my boat.