I agree that it does not hurt to leave them plugged in. I have left mine plugged in overnight may times, I also have left it plugged in 24 hrs a day for a few days in a row. I had plugged it in, and we had been busy and not driving the truck and I just did not worry about unplugging it. I look at it this way, I know several big truck drivers, and in the winter if it is not running it is plugged in.
I have also been known to leave it plugged in, in the somewhat heated garage, when it is really cold outside, just so the I have heat when I get going. The electricity bill is high enough, that even if I do keep it plugged in all the time, I do not see an increase in the bill.....