A spat with a family member?
I'd say tell another family member. A general rule of thumb is to tell the person who can talk to the family member you are in disagreement with. Someone this family member can listen to will also likely be someone who will keeps things said in perspective and de-escalate comments said in anger or made in exaggeration knowing the personalities of the people involved. I'd say tell another family member over telling a spouse or a friend- you might feel responsible if your spouse can't get over what that family member did to you long after you have.
A spat with a friend?
Hmmm. I'd say don't spread the poison to the other friends, keep it closed and don't expect/even discourage mutual friends from taking up your fight and acting funny towards the offender. Keep the he said she said to a minimum by having the refereeing friend and both of you in one room while resolving issues.
A spat with your partner?
Tricky. In general circles keeping it between yourself and your spouse is the rule of thumb. This is an opportunity to exercise the motto that there is nothing that you cant resolve between you and if problems are always resolved for you, you loose the opportunity to learn to tide over your issues together. The problem here is usually in not waiting for the ripe time to address heated issues.
I've noticed that at the right time and with the right approach you can get your spouse to understand how they hurt you and tell them how you felt about your experience of the disagreement between you two. You know better how to persuade and negotiate and what to leverage to upturn an argument than the referee who might miss the nuances in your relationship and attribute their experiences to yours.
I think generally and 99.9% of the time this should work but when it gets to that matter that is the 0.1% , go get that referee!
A spat between your spouse and their sibling?
Stay out of it. hahahahaha! Ok this is a borrowed opinion from someone who maintains that when rifts are mended, your comments might come back to haunt you. But here you probably want to provide your spouse understanding and help him/her keep the peace and unity of his/her family in view while taking a decision but for the most part, keep away from saying negative things about the offending family member and keep to suggestions- let the spouse decide how he/she prefers to handle his/her family.
A spat with a colleague?
If its a subordinate, try to resolve it. If it gets beyond what you can handle, channel it to human resources. I find that in the work place, following official channels of conflict resolution are more effective at passing the message to the offending party that they are in a corporate environment and have to play by the rules.
A direct effort at letting the offender know what you are unhappy about, followed by a report to a line manager and after that a notice to human resources is the recommended approach. Otherwise it will fester, others will copy the bad behaviour or the subject becomes fodder for office gossip and escalates into a mountain.
On a final note, keep all personal things personal and out of the work place, it will give you a sense of self respect when a colleague isn't using a personal matter to insult you at work.
So before you open your lips to vent, think first- who should I tell this?