The answer to how you file is how you use the files. Do you have more occasion to retrieve files by customers' name or by date?

If it is by name...then alpha. If it is by name but date is also very important...I would use a file label that has both on the label...

Jones, Susan 2007, July

The order...most recent or oldest...again...which are you more likely to use?...probably most recent... Whatever is most likely to be needed should be easiest to retrieve (on top!)