Whether you are staying in a hostel or a hotel I find it important to keep a couple of combination locks on you. I would suggests those over the ones with a key because the last thing you want is to lose the key. It is important to get TSA accepted locks so that you don’t have any trouble while going through the airport. But I’d say it’s not just important to keep them on your bag while you aren’t in the hostel or while you are sleeping, but also important if you bring a day pakc out with you for the day. Since most backpacks open from the outside where you can’t see (though I’m seeing a trend start to change that, yay!) if you got your bag locked up, it makes it harder for you to get pick pocketed. Last thing you want if for your camera, phone, wallet, anything of importance to get swiped from you while walking down the street or on a crowded train. So keep those locks on all your zippers!