I flushed mine out several years ago and haven't been able to blame any problems on doing that. Maybe I got lucky, maybe not. Any oil that goes through the oil cooler *has* to go through the filter. Unless you have a filter with a bypass inside it, any debris that works its way loose will get caught there.
When I flushed mine I used a garden hose, putting as much water through the cooler as I could, going in both directions. I caught samples of the water coming out, and once I started catching samples with no visible material I flushed it for another 5 or 10 minutes (both ways) and then figured it was clean. Drained it, blew it out, filled it up with rubbing alcohol a few times, draining and blowing it out each time, to make sure it was bone dry and no water was sitting in there.
Worked for me.
John