The narrator's use of "we" suggests that although one person may be telling the story - she considers her opinions to be those of the majority. The actual identity (even gender) of the narrator in "A Rose for Emily" is never disclosed - however it is obvious that it is someone who lives in the town, knows the Griersons (but likely not on a very personal level), and is in the know on all the current town gossip.
I like to suggest that the narrator is female, simply because stereotypically, the personality fits that of a neighborhood (female) gossip. The use of "we" also suggests that everyone is talking about Miss Emily and the Grierson's behind her back. It paints each strange event in a "Miss Emily vs. the town" type of conflict. Of course the narrator wishes to remain anonymous - consider the things she says about Miss Emily and her family. All of it is in the spirit of mean hearted gossip. It is very obvious from this perspective what the town thinks of the Griersons - which isn't much. And there is safety in numbers. What better place to hide as an individual than with a crowd of people who agree with you?
It still happens today. Whole groups of kids will make fun of just one - but this picking on would likely never turn into a one-on-one issue. It is always easier to be mean and rude when "everybody's doing it." That is exactly the message this narrator wishes to convey.
In "A Rose for Emily," the pronoun "we" refers to Miss Emily's neighbors. I believe they are people who have lived their lives in the same town and have become accustomed to Miss Emily's idiosyncrasies. Those who lived before and remember her father, family and early history provide information that members of contemporary society would not have been alive or old enough to witness.
The "we" that speaks is a part of the culture in which the present day story takes place. Because Miss Emily's neighbors are used to life in the South, some of the things they mention would not raise their eyebrows, but simply provide background that is steeped in seemingly trustworthy observations which outsiders could not notice or appreciate.
Eventually, these same casual observations provide information that cause the reader to ask the important questions and draw the essential conclusions at which Faulkner hopes the reader will finally arrive--so that the story's classically shocking twist will not be lost or overlooked but, instead, cause the reader to pause in alarm, with a sickeningly sweet horror that repels and arrests our attention at the same time. The final sentence leaves us to deal with the truth of the moment, where all of the clues leading to this instant gain brilliant clarity and chilling purpose.