I don't consider myself to be much more than a novice at WitE, but here are my two cents.
I think it is worth while for the Russians to put up a fight along the upper Dnepr River (from the bend at X80 Y53 to X81 Y69).
The major rivers are pretty defensible in this game. While the Germans do arrive quickly to the Dnepr, their tongues are hanging out a bit and they are probably a turn or so ahead of their supply lines and infantry support.
In the little bit of experience I've had, defenses along major river lines cannot be broken without well planned and supported attacks including infantry and air support. Hasty attacks by panzer spear heads won't get the job done. I know, I've got little digital panzer divisions with the scars and casualties to prove it.
If you are going to make a stand along the Dnepr you need to do a couple of things.
1) Include Vitebsk and the woods hexes south of it in your planning. If the Germans breakthru there, your Dnepr stand is pretty much finished and you then have a severe risk of getting your river defenses rolled up and surrounded.
2) The earlier you can get guys digging holes (fortifications), the better. The riverline plus 1+ level forts is much improved.
3) Pay attention to HQs positioning. Also look at the use of a checkerboard in a second line of armor units set to reserve mode.
The big issue is when to give it up. If you make a big investment in troops along the river, they are at risk. You gotta know when to quit and retreat. Having a smattering of units behind your main line can prevent a complete encirclement. The movement costs the Germans suffer to cross major rivers, plus making them move thru Russian zocs and attack units can help keep them from completing the encirclement.
Behind the Dnepr is a minor river plus woods that can be good to fall back to. I have sometimes put routed units in those hexes to recover. There they can build some forts that Dnepr defenders can fall back to. That line won't hold for long. But the idea is to make the Germans burn the summer and take as many turns as possible to get to Moscow.
Again, I'm not an expert -- but hopefully this makes sense for some players.