The best answer to this is like I said in my post, talk to your agent, let them do the home work and find out how much their underwriters will cover you doing work outside your scope of trade, it's always better to find out before hand than after, we must remember we live in a sue crazy world these days, we are our only protectors and we can do this by getting the knowledge before something happens because after is just too late.
Trust me homeowners will say all kinds of things to get you to do other types of work so they don't have to find someone to do it or do it themselves, and it can be very tempting in this economy to make a little extra money, but when things go south guess who they will blame, and remember one thing you can't sign off on liability even if the home owner gives you a signed sheet of paper saying so, in most courts of law it is worthless.
The thing here is each one of us will be under different laws in different states, so there is no one answer that fits for all of us, this is why it is up to you to find out, and seek for the truth not just what someone told you, I seen many burned by trusting hearsay then finding out to late that what they were told was wrong, I caught our company lawyer one time giving us some misdirected advice, lets just say he didn't last too long when my boss found out he was setting us up for a fall, you would think why would he do this, well the short answer is greed since he would stand to make some money if we got in trouble, so my advice is do your own homework, be your own protector.