About renewal, there's a script file (RegionsDecisionsRenewal.sct) in the ..\Events folder which controls when RGD's regenerate.
About using RGD's, I've probably looked into those which raise the civilization levels, and build tracks/trains--they are the same thing--, and roads more than anything. Depending on your strategy, they can be locally useful.
One place I like to build a road is in the 'bow-tie' region between Jefferson and Springfield MO, which I start building during the winter of '61-'62. This increases the supply flow into Springfield in '62 making it a much more stable base of operations. But of course you have to guard the construction for 16 turns for it to complete, which can be a delicate situation.
I've also built a road down from Lexington KY toward Knoxville a couple of times. You don't need to build it all the way, but it allows you to use that avenue for a forceful advance on Knoxville and Chattanooga, which can put the South into a predicament. It won't really get you into Chattanooga much faster, but will pull some focus away from the western Tennessee bottleneck.
As the Union player, Partisans can be a fun distraction--fun for you

--in middle Tennessee. There are many regions where you have enough loyalty to use the RGD, where the South generally has no eyes. Attacking depots and rail lines can be quite useful. Cutting the rail line between Tennessee and Virginia at the right time can have a great affect, but generally only temporarily. Also just riling up the CS player and forcing him to expend more troops to garrison the area is to your advantage. Eventually they need supplies though and have to turn into a 'friendly' town, which they capture and which then betrays their location. But with a little luck and some skillful handling, they can form a minor secondary front for a while, and definitely add some interesting variations to a game focused mostly on big armies with huge power.