Only time I've ever had an issue was on an older corolla, some water snuck down past the spark plug wire boot and was causing a misfire on that cylinder since the spark didn't make it to the plug. Quick shot of dust off, and a little wd40 fixed it.
Every couple months I clean under the hood of my Nissan truck. I spray it down with degreaser pretty good, wipe the dirty parts with a cotton towel, and rinse with garden hose on low pressure. I don't use a pressure washer. I recently bought an air compressor, so last time I used it to blow out all the water from electrical connections and the nooks and crannies, it really helped out.
Follow with armor all or whatever tire shine you like that doesn't contain teflon (MAS doesn't like that)... That's about it. I always get compliments on my engine bay. If you keep it clean, it's easy. The hard part is if it gets really dirty.