If you plan to stay in youth hostels, in short, the answer is yes.
Most hostels now require that people sleeping in their beds use sleep sacks. Most youth hostels will give you a sleep sheet or a regular sheet, but it will probably cost you extra to use it. But because some youth hostels don't have them, it's always a good idea to bring your own.
Sleep sacks are easy to make. They help keep the beds clean, which is why hostels make us use them. It's good to have clean beds.
You can also choose the materials for your sleep sack, which is a plus. If you're going on a trip in the summer, you might want to wear something thin and smooth so it doesn't stick to you if you sweat a little. You can choose a thicker material for winter that will keep you warm on cold nights.
Basically, all you have to do to make a sleep sack is fold a sheet in half. Once it's lined up right, you just sew the bottom and half of one side to make a sleeping bag that looks like a sheet.
A lot of travellers have asked about sleeping bags and if they should bring them. In my opinion a sleeping bag is a bad idea. It's bulky, hard to wash when you're in a hurry and don't have much soap and water, and many hostels don't even let you bring them in for reasons of personal hygiene (you wouldn't want to sleep next to a guy who had been living in the woods along an interstate for three weeks in a thick, sweaty sleeping bag).
So, to answer the original question, if you plan to stay in youth hostels, you should bring a sleep sack with you. Sleep sacks are good for you, the hostels, and everyone else on the road.
I think you should include a sleep sack on your list of things to bring. You won't regret it.
