You are notified when you reach the limit by a message stating how many down votes you've made and what your current karma is. Until then I imagine you should not let this limit influence you:
I disagree, I want to reserve that power for where I see fit to apply it. I would prefer to calibrate my behavior, so that I won't be left out unexpectedly.
If you reach the limit and encounter a post you really want to downvote you can always un-downvote a previous comment that you dislike less. That would be easier if the 'disliked' link showed comments as well as top level posts.
Our friends at Tricycle will push through a zero-based karma system (no self-voting possible) sometime this evening. At present this will only cover future posts/comments - they may go back and revise history some time in the indefinite future (or not), but apparently that would be overly complicated for now. We'll see how this works.