In short, if you plan on staying in youth hostels, yes.
Most hostels now require travelers to use sleep sacks when sleeping in the beds they provide. Youth Hostels will often provide you with a sleep sheet or regular sheet; more than likely this will cost extra money to use. However, because some youth hostels do not have them on hand, it is always a good idea to bring your own.
Sleep Sacks are simple really. Hostels have us use them because they help keep the beds clean. Clean beds are good.
With sleep sacks, you also have the advantage of being able to chose your materials. For summer journeys, you may want to go with something very thin and slick so that it does not stick to you if you sweat a bit. For winter, you can chose a thicker material that will help keep you warm on cold nights.
To make a sleep sack, really all you need to do is take a sheet and fold it in half. Once you have it lined up right, you just sew up the bottom and half way up the side to create a sort of sheet version of the sleeping bag.
Many travelers have asked about sleeping bags and whether or not they should take them along. In my opinion a sleeping bag is a bad idea. It?s bulky, hard to wash when you are in a hurry and have little detergent and supplies, and a lot of hostels do not even allow them do to personal hygiene reasons (you wouldn?t want a guy who has been living in the woods along interstates for three weeks sleeping next to you in a thick, sweaty sleeping bag).
So in answer to the original question, yes, you should take a sleep sack with if you are planning on staying in youth hostels. Sleep sacks are good for you, they are good for the hostels, and they are good for everyone else who is on the road.
I recommend you add a sleep sack to your list of things to pack. You won?t regret it.