...at least that is my take on how to handle Rohan with the Shadow.
1) If you attack early, you will either have to spend a lot of resources to take Edoras & Anorien, OR you will be faced with , an unchecked Rohan military build-up. If you don't Rohan will have a 10-unit strong army in endgame, with lots of possibilties (Helms Deep, Orthanc, Lorien, Minas Tirith, Morder), distracting Shadow military strength.
2) By not attacking Rohan, you might lure the FP into spending dice on the Rohan military track. Great! Let them waste those preciuos dice. Make sure you attack them before they are able to put a unit into Helms Deep.
3) In the endgame (just 1-2 turns left) it doesn't cost you as much to lose Saruman from an Ent Attack. Thus you can exit Orthanc with great force.
4) The Ent cards become a burden to FP hand. Will he keep them or *gasp* have to discard them - due to hand size limit....!
In the expansion: 4) The Ent threat will never have to materialize.
Note: I do not advocate against mustering Saruman (should be done ASAP) or against taking HD at all (HD should be taken in just about 90-95% of games! It is just that I think you should wait a bit before attacking. Take 7-8 VPs in other locations. THEN wipe the Rohans out of existence
Well, these were just a few reasons , taken from top of my head. So, is this common practice, or do you think my thoughts are flawed in some aspect?
I would say this is a common practice. Alot of times I don't bother with Rohan at all and at most only take Helm's Deep for the win and not touch Edoras. Alot of FP move the Edoras troops over to the Fords to blockade better.
I definitely agree though, that Rohan and Gondor can not be poked early, since putting them to war soon will have the SP worrying about large FP armies in the center of the board...
One question still remains for me though: if I get Eomer, Son of Eomund early, I don't know whether to put him in Edoras or Helm's Deep. On the one hand, improving Helm's Deep's [1/0/0] to [1/1/1] is unlikely to help it much - its still dead from a single Deadly Strife. On the other hand, putting it in Edoras seems very passive.
If I draw Wormtongue early, I'll consider an early attack on Rohan (with lots of Voice musters), the goal being to capture every settlement in Rohan to prevent them building up later (and of course the 3 points are nice). And maybe have enough orcs/wargs/dunlendings left over to help siege MT. But otherwise Rohan is best left until the end, if it's attacked at all, imho.