The one thing you must understand about open clutch covers, they were originally designed for and used on the track. By keeping the clutch open, they were able to keep the discs cooler, which in return increased clutch life. As with most things race inspired, it was just a matter of time before everyone and their mother was running open clutch covers on the street.
For street use, you obvioulsy have certain things to worry about. Getting the clutch pack wet won't really stop it from "working", but over long exposure to constant water or debris (in example: sand or mud) will definately lesson clutch life.
Washing your bike isnt a problem, as long as you start the bike and go for a ride after and NOT let the water to sit on the open clutch pack for a long period of time. The OEM components (springs and hardware) have also been known to rust easy. After you wash you bike, simply run the engine, let it heat up a bit, or even better, go for a ride. That will take care of any water left on the clutch / pack.
Last but not least, depending on how "open" the clutch cover is, you obviously need to be a little more careful with your riding gear, tools, and fingers when the engine is running. Common sense goes a LONG way here!