I have uniden bc355n installed in my truck. this 1/4 wave antenna is 20.5 inches. i listen to airbands, 2 meter ham and 150-165 vhf range. Can i leave it uncut or what would be the best length in middle to best suit the best frequency range.
Yeah, don't change anything. 20.5 inches should be resonate somewhere just below 140MHz. That's pretty dang close to the middle of the band you are listening to.
Anyway, it's unlikely you'd notice any difference by trimming the whip. If anything, tune it for where you transmit (if you do), otherwise, just leave it as is.