I use SBB's here in Western Washington. I also keep them open all winter, helps keep moisture out. I've had no problems with the number of bees decreasing. The only thing I don't like about screened bottom boards is keeping them clean. That is until I discovered the SBB's from Walter T. Kelley. From the back of your hive you can not only pull out the plastic sheet for doing mite counts, but you can also pull out the screen it's self, dumping all the debris out. Makes it really nice, especially if you had a harder than normal winter like we did this year and have lots of dead bees blocking the exit. That way you don't have to tear the entire hive apart or move double stacked deeps off the bottom board. :-)
Sean Kelly