I found that the freshness, quality, and grind of the coriander makes more of a difference than when it is added.... that, and the experience which comes from making lots of Wit.
When I first started out making Wits, I would sometimes add coriander to a hop bag in the fermenter for several hours to a day or two to make up for lack of flavor from adding to little or stale coriander during the last 10-15 min. Really have to watch it since the flavor can become overpowering before you know it and I prefer just a nuance of coriander to balance out the other additions. We grow our own these days and the difference in aroma from store bought is amazing...