I work in an office full of Mac users, and I'm among maybe three people who don't use Macs for work at the company.
All of the non-Apple monitors in the company are not correctly calibrated and lack colour profiles (and are by default not configured correctly either).
I recently bought a colorimeter, and calibrated all of my displays(including my external displays); after doing this I realized that the colour on my monitors most closely resembled the default colour response on the MacBook Pros in the office.
My colleagues(who admittedly didn't understand colour calibration at all, even the designers) are under the impression that Apple profiles and calibrates the panels at the factory(and presumably ships custom profiles for the factory state of the panel).
I think it's probably cruder than that. It's likely that they ship one profile for an entire family of panels, which gets it closer but doesn't account for time and manufacturing variances.
Get a Lenovo huey or a Colorhug, correct your monitor, and you'll see what I mean.
On the web, you don't really know what you'll run into. You should aim for a level of contrast so sharp that it borders on uncomfortable, so that when the contrast between your colour selections is less than you thought, it'll still at least be legible.