Actually C-14 levels have varied over history, so we already need to calibrate C-14 readings with other measuring sticks, such as tree rings.
That calibration would catch any possible variation.
Incidentally did you know that C-14 dating no longer works? The reason is that we've released a lot of carbon from fossil fuels that is very low in C-14. As a result fresh organic matter today has lower C-14 levels than organic matter from 100 years ago. Odd, but true. (And don't think that the Creationists haven't been all over that fact.)
My general point was that one can make a mathematical model that takes into account the historical value of the ambient C-14 to C-12 ratio, plus the historical value of the C-14 decay rate.
C-14 dating isn't used for dating recently-dead things, but not for the reason you suggest. The real reason is that the half-life of C-14 is about 5730 years. Not much decay happens in 200 years, and the little that does happen is difficult to sort out from measurement errors.
Other techniques are used for dating things that died recently. Watch CSI for some clues :D
That calibration would catch any possible variation.
Incidentally did you know that C-14 dating no longer works? The reason is that we've released a lot of carbon from fossil fuels that is very low in C-14. As a result fresh organic matter today has lower C-14 levels than organic matter from 100 years ago. Odd, but true. (And don't think that the Creationists haven't been all over that fact.)