Depends.
You can run both if you want. Presumably you got the
Blue Iris server so that you could have it manage recordings, as most people do. In that case you could turn off the alerts within the cameras, and disable their motion detection.
However if you have any cams recording to SD cards or NAS drives, that can make a nice backup to leave their own recording/motion detection enabled.
Alternatively if you like the camera's motion detection settings better than you like Blue Iris' motion detection, you can configure Blue Iris to poll the Hikvision cameras for their alert status. This would allow you to disable Blue Iris' motion detection while still having Blue Iris record on motion. This could be desirable if you are using line crossing detection well, or if you have cams with PIR sensors. If you are interested in that, here is a blog that explains how you can set up BI to monitor the Hikvision camera's alert status.
http://www.petermcannell.com/using-ip-camera-based-pir-sensor-within-blueiris/