What angle are front rotors of tandem helicopters tilted and why this value?
The forward tilt of the main rotors with respect to the fuselage is not related to the tandem rotor design, but to a combination of fuselage lift and the need to also provide thrust with the main rotor.

First off, a review of other tandem designs shows that those not related to the CH-47 do not always share the forward tilt:

Bell XHSL-1 prototype



Filper Research Beta 200A



McCulloch YH-30



Piasecki H-21B



Yak-24



And looking at non-tandem helicopters, we also find a similar tilt:

Sikorsky CH-53



NHIndustries NH90



Mil V-12



Mil Mi-26





The reason is twofold:

A helicopter obtains its thrust from the same place as most of its lift: from the main rotor. This means that in forward flight the main rotor will always have its tip plane slightly tilted forward to counter the drag acting on the airframe.

Furthermore, large helicopters like the tandem designs you ask about experience significant aerodynamic forces on the fuselage at high speeds. To mitigate these, the main rotors are installed at an angle so that in cruising attitude the lift component from the fuselage will be positive. You can see it more clearly in flight:

NHIndustries NH90 in flight



Sikorsky CH-53 in flight



Boeing CH-47D in flight



I could find some corroboration of the fuselage lift effect in this Boeing report which includes a lift curve for the fuselage attained via wind tunnel tests. As you can see $\alpha_{zero-lift} \approx 4.5^o$



As to what is the exact angle, that depends on the design and is ultimately optimized after extensive testing, if deemed worthwhile.