This days it makes no different in most cases.
If you look in the URL field, you will always notice that the http:// is their automatically. People are still accustomed to butting www. in front of the domain. Go ahead and try it. search on a domain without the www. Then without the http:// then without both. Use in most cases will still be taken to that website's.
As for me, if I want to go to the Sears website I wound just type Sears.com. If that didn't work for some reason, then I would type www.sears.com because the htp:// is already apart of the domain you are typing in. To check this, just put your cursor anywhere in the domain and move it all the way to the left. You will see the http:// as part of the domain.
One comment - if the domain is https:// then the http or any combination will not work because the https:// is a secure domain.