Actually, with IG-88, my young self assumed/invented a back story for him where a company manufactured these droids and clients could sort of rent them. For example, I have some idiot that is making my life miserable and I need him out of the way, this company would look into the situation, and if they deemed it a job worth taking/something that wasn't going to get them in more trouble than it was worth, then they'd sent one of their programmable hitmen to deal with the situation for me.
As far as I know, there is no real canon backstory for IG-88 beyond the fact that he is somehow and bounty hunter and an Imperial model Assassin Droid. The EU backstory for IG-88, while being a very amusing and fun to read IMHO, is absolutely and utterly ridiculous. The EU story is that he became self aware, killed his makers, made copies of himself inside other IG droids, and started working as a bounty hunter to pass time until his plans for universal domination came into action. Oh yeah... and there was that bit about him transplanting his conciousness into the Death Star and essentially becoming the Death Star.
So yeah, I guess if we take the EU story to be canon worthy, then IG-88 was an exception to the seemingly rule of droids having owners. But from what we see in the actual movie, it is reasonable to assume he has an owner, but the owner didn't deem it necessary to travel across the galaxy meeting with Vader and hunting the bounty when he could simply sit on his rear end and let his robot do it for him.