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Always thank the hosts before leaving. I would consider it rude to slip away. You don't need to say goodbye to all the guests - just hunt out the hosts and say thanks and go.
The only time I ever 'slipped away' was at my own wedding reception. I was a little bit tipsy - me and hubs just left the party and went off up to our bedroom. It was easier than waiting for everyone else to leave, and quicker than having to go around the whole room thanking/saying bye... So we just did a runner
Since having kids and paying for sitters, we tend to not stay out till the wee small hours any more. Round here though, lots of people have family to park the kids with so they stay out much later Is it more polite to (a)find the hosts to thank them and say goodbye (but risk drawing attention to the fact that the party's breaking up- recently had someone say "oh no, you're not going HOME?) or (b) just slip into the night but be sure to thank them after then party?