I always put my dew shield on, even in the summer, and particularly at public viewings as it helps to keep the fingers from little kids off the corrector! I find the shield will keep dew off for maybe a couple of hours if you are looking away from the zenith, but if you point the scope high it will dew up in minutes. I remember one night I was looking at something and swung around to something on the other side. In the process the scope had to swing high for a few seconds before pointing down again. When I looked in the eyepiece I couldn't see a thing. The corrector had dewed up in the time it took to slew over
One of the next things on my list is a heater...