I guess if you dont declare them then the insurance company can refuse to pay out and technically youre driving without insurance
I wouldnt get this too out of perspective though, if you were to have a rear shunt or if you shunted your car into someone else, I very much doubt an insurance investigator would come out and examine your car (they never when i was rear ended twice). However, if you were involved in a serious accident involving injury and/or a write off then an investigator would assess the car, notice the tinted windows and notice theyre not declared.
You might also want to double check the law (ive read conflicting info on the web about this) as I believe tinting forward of the B pillar isnt allowed anymore.
Similarly, if you had them tinted illegally and you were in a serious accident then youre in the sh1t.