personally i wouldnt do it, but i know people who have. equally i know people who have done loads of mods to a car and have the car declared as standard.
pointless imo as there argument is "im paying 1500 to insure it i cant afford to pay an extra £300 for all the mods" so basically youve wasted 1500 on insurance that isnt even insuring you

personal preference really, i know two people who have had maps and one crashed his car and the insurance company paid out and didnt scan the ecu and the other hit a wall at relatively low speed and again the insurance company paid for the damage to his car and the wall as they didnt scan the ecu.
so not all companies scan ecu's but to not declare it is playing insurance russian rulette, youle never know if there gonna scan it or not untill its too late.