If you've got something to say, say it. If you've got nothing to say, keep quiet. Sure, if you don't tweet anything for weeks and weeks, you're likely to get people unfollowing you. But equally, if you just tweet 'filler' day after day then people will unfollow you a whole lot quicker. People are MUCH more likely to unfollow an account that is actively annoying them by posting rubbish every day than one that doesn't do anything at all.