Having found mostly small urban caches the main thing that stops me signing logs is just that there's either no space or the log isn't in a fit condition to sign, usually because it's wet (very common at the moment). I've always understood the rule is that if you can't sign it take a photo - making sure you don't give the location away too much of course.
Incidentally, I always recommend pencils rather than pens for Geocaching as they write vertically... handy when the only hard thing to lean on is a tree or a wall