I may not be posting this in the proper place but since I started out as a mapper and was sort of converted into a coder and designer, I felt this is the best place for it.
For anyone who wishes to begin a new mod and especially one that you would like to easily share with others, I suggest you read and become familiar with How to use the -mod:my_mod commande line switch
I still think that Myrddraal's tutorial on creating a new map is a must read and the way he has you set up the provincial mod folder works fine so long as the only thing you are doing in editing the map and changing the names of the provinces, cities, and changing faction ownership and such. But once you begin editing all the files in the data folder and changing ground textures and editing climates, and making new faction names and symbols and new models etc, then you start editing things that once changed you will need to either send your entire data folder off or remember which files you editied to send to others.
If you begin with a my_mod folder early on, this will make the processs of packing everything up to share with others so much easier. And it has the added benefit of not messing with your vanilla campaign.
I never even heard of the my_mod folder until last week when I was telling Roman Man#3 that I would send him the entire data folder because i had made so many changes to the mod I was working on. He laughed and told me to check out the my_mod folder tutorials. It took me about three days working late into the night to get all the files I had edited, re-path all the units and textures, and test it, but when complete, I had a 160mg.rar file instead a 2G Data file that was our entire mod. This included everything from the splash screen, to the faction selection icons, to our new models, and all my climate edits.
Also, I now knew all the files I edited and it was easy to find them if I had a problem. And I could send this folder out to the other members of the team and easily update them since they only had a few folders to find things in rather than having to become familiar with every aspect of the RTW Data folder.
Anyway, all this to say, creating a my_mod folder and placing all your changes in there will save you time and be more convenient in the long run. And now that I've begun using it, I can't think of any reasons not to.