My mum used to say "If you can't say something nice, don't say anything."
I think that this applies to online poker chat too. If someone wins a pot against you, regardless of how they do it, throw them a 'nh' and leave it at that or say nothing.
The only exception to that is if you know the player really well, and I mean that you both feel you are buddies not that you have played against them lots of times, you can add to it a little. Something like 'nh you idiot' and don't forget to add a smiley. Very important creature, the smiley.