I have personally found that you are almost always better off running even front and rear shore tires on newer cars. I am talking about cars manufactured in the last year or two for foam tires. In fact most cars seem to be "optimised" for 40 front and 40 rear.
My experiences is that tires under 40 shore tend to make the car wander badly in the straightaways and also make cornering inconsistent.
I would always start with 42/42 or 40/40 and tune from there with the chassis unless it is clear that 40/40 is too hard. I very rarely have used under 40/40 and also I find that going softer does not yeild that much more useable grip.