I think you can pretty easily correct for discrepancies in color temperatures on an enlarger if all of the light in the image is the same color temp, ignoring some color crossover in the shadows/highlights (which might not be too bad). You will run into problems if you have low CRI fluorescents mixed in with daylight or tungsten. Again, I've found the Portra films to handle tungsten lighting pretty well and mixed daylight/tungsten environments.

As far as MRC goes, with the exception of one lens, all of my lenses are either 39mm or 46mm. And my main three lenses are 46mm; the 39mm lenses don't get used anywhere near as much, and rarely with filters. So, I've been building a nice set of 46mm filter that I share on my lenses. As a result, for the extra $10 for MRC, I get it. If you want to collect them all in a bunch of different sizes, then maybe don't spring for the MRC.

Here's a link on Wratten numbers. The 80 and 85 series are the workhorse for color temp conversion.
http://en.wikipedia.org/wiki/Wratten_number

I also find the B+W filter handbook to be a nice resource for filters:
http://www.schneiderkreuznach.com/pd...r_handbook.pdf