What is the "right" rate at which my papers should get accepted (to computer science conferences)?
To a first approximation, promotion depends on top-tier publications and nothing else. If career concerns are important (and they always are) keep submitting to top-tier venues (as long as you believe the work is good), even if it takes several iterations to get in.  

There's so much variability in what gets accepted that it's best not to take the outcome of any single set of reviews too seriously.  If you start to see a consensus among reviewers as to why they recommend rejection, then of course you will need to address that.  But otherwise, don't lose hope and keep submitting.

Context: I'm a social scientist.