The preferred way is to choose either www.example.com or example.com and let only one of them resolve, while the other should issue a 301 ("Moved permanently") back to the one that resolves.
The easier one to set up is to prefer the www version since it can be done with a simple CNAME and many DNS providers will offer a free automatic 301 forward for the non-www version. (But tastes differ on whether the non-www version looks better.)
Letting both versions resolve would give issues with Google indexing multiple versions of the same page, and reduces the ability to cache properly.
I'm looking to park a few low-traffic charity sites there and 'Set it and forget it' (like Ron Popeil). I think having both with and without the www working is a higher priority than SEO, so I'll have to compromise and set A records for example.com and CNAME for www.
If anyone figures out a better practice please leave a comment. My preference is no www (with www 301-ing) plus the CDN benefit, but this probably isn't possible.
The easier one to set up is to prefer the www version since it can be done with a simple CNAME and many DNS providers will offer a free automatic 301 forward for the non-www version. (But tastes differ on whether the non-www version looks better.)
Letting both versions resolve would give issues with Google indexing multiple versions of the same page, and reduces the ability to cache properly.