There is no standard, but the www. is not actually required. My host allows me the option of allowing/blocking access to either of them, however it makes sense to me to allow access to both.
In fact, what I did was to set it to redirect all visitors to domain.com to www.domain.com in my hosts control panel, just because I prefer the look of a www. in front.
There is a cinema web-site that I sometimes check, but without the www. all of the images fail to load which makes it look completely broken and there's also a PC hardware site that will simply fail if I try and access it without the www., very frustrating.
Generally I will access all .com, .net and .org sites using the www. because there is a keyboard short-cut for my browser that will add a www. and a .com/.net/.org onto anything I type in, but for .co.uk sites I usually omit the www because it's less to type in. However, I don't see any reason to turn away users because they left out 4 characters.