If one of the couple is a blood relative, I usually refer to them first. (when speaking of them to someone)
But sometimes a couple's names, when spoken, flow more smoothly in a certain order- like, Harold & Maude, for example. Or Liz and Dick. Samson and Delilah. "Delilah and Samson" just doesn't trip over the tongue as smoothly.
As far as personal correspondence goes, (sorry, Miss Manners!) I was always taught that the person actually doing the writing put their name before the partner. That way, my words are my own and recognized as such.
On formal correspondence ( concerning both hubby and I) I sign as Mr. and Mrs. - my husband's name first.
When I am being silly, I sign from 'him and me'. Or use something like: Mr. and Mrs. Snorklebutt, Mr. and Mrs. Potato Head….. Goldy and Balthazar…..whatever.
( to be clear, those are are not our real names. HA!)