I left it loose, I just use a colander after cooling to strain the majority of the pieces. I ground the coriander to a dust this time around though, and that led to my siphon getting stuck constantly when I started getting towards the end of racking it to the keg. So, if you just crush the coriander, you should be fine. If you grind it though, I'd definitely find a way to keep as much as possible from getting into your fermenter, or you could just wait a while in the fermenter for everything to settle out and compact a little. I like my wits fresh, so next time I'm probably going to use a bag in the boil next time for the coriander.