At home I alphabetize by utility/catagory, use staples frequently until they are so thick the stapler jams and I have to start a new file, and I put the most current in the front since I'm most likely to be wanting to look up last month or two utility bill or something similar.

At work we alphabetize by owner's last name (if hyphenated we go by first of the two last names), most current file gets put in the front (client files only), and we use staples (paper clips have been known to suck up someone else's file and then it gets filed with another client's folder and may never be found, staples don't do that). Our computer system can look up by animal name so we don't use that for our paper filing method only the owner name.

At work our invoices/utilities get filed in whatever system the longtime receptionist wants to do, and it confuses me because she puts the most current invoice in the BACK. I'm also likely to find the GTE/BELL/VERIZON/Sprint invoices all in the same file listed under "T" for 'telephone'. Gets kinda confusing to me, but she's been doing that way for years and always seems to know where to find the invoices we need.