If i remember correctly, it has to do with the frequencies(highest). I read it in the manual when i had my 796 but i can't remember.
The manual can be downloaded here if you don't have one. http://www.uniden.com/pdf/BC796Dom.pdf
When trunking the squelch should not make a difference.
I normally suggest a middle settings (just in case ur also going over conventional freqs in another bank you wont miss them, or on the opposite side get static)