I have had good experience with Crucial and then OWC, which is less expensive and never, that I know, gave me a problem I could attribute to the RAM. I’ve brought memory for Powerbooks, Macbook Pros and Mac Pros.
Theoretically any RAM will do; I remember that Powerbooks (and PowerPC in general) had a “different chip” and not every memory DIMM would work (despite being the same type/speed), but my memory is lost after all these years of Intel Only Macs.
All in all, as far as I can remember, the “Apple Certification” was due to the reliability of the memory. I know that Crucial/OWC and some others are Certified, but that’s because certain machines (i.e. Mac Pro) expect to have a heatsink in the modules, among other things.
It’s usually safe, but don’t pay “more” just because of that, because it tends to be the same thing.
In the Macbook’s case, there’s no heatsink involved (no room!) so pick something from newegg or those sites and you’ll be ok.