Some shutters are better sealed than others, but full-frontal nudity (no lens on the body) will expose the shutter to a higher level of light than through a lens. Not only is it brighter, the source is larger and not as controlled as you would have with a lens on it. I just turn my body around to shade the camera/lens and have never had a problem.
Holes being burnt through the curtain is an issue in some cases if you leave the camera pointing the wrong way for a long time (lens facing up on a table outside on a sunny day).
B2 (;->