If you're developing for a CentOS server, I'd say you'll have to do some final testing on a VM in 95% of all cases. Even a CentOS laptop that you use for production (and maybe a little fun) will have some major differences to a server deployment. And that's assuming that you're enough of a glutton for punishment to be running CentOS 5.6 on a laptop.

Even if you're running Ubuntu 10+ on your laptop (a somewhat more sane assumption), you'd want to test your code on something closer to the "metal".

And honestly, (L)AMP (for normal values of P) won't be a problem on OS X. The usual frontend stack doesn't have major differences, maybe apart from a different default versions of the software (which are easy enough to install). Once you get closer to the system-specific stuff (when it get's more unix-y or network-y), then care about the differences. Not exactly the case for most web scripting stuff…

It's really not that bad. Homebrew may have fewer packages than, say, Debian's apt repo, but I still don't use 99% of the packages in either, and both cover everything I need.

Consider what exactly you'll be using in your stack, and go from there. I've had zero problems over the years using OS X for Rails, Python, node.js, and C++ development that was seeing deployment on Linux (typically Debian or Ubuntu, sometimes CentOS).