I used to offer a sub service but I would never allow them to tie me up, no matter how long I've known them. By the very nature, a Dom is someone that wants control over another and gets off it. They are turned on my inflicting pain, humiliation or degradation. It is possible that the situation could get out of hand if you are tied up and they are enjoying your struggling a bit too much.
I am an owned sub, a proper masochist and my Master is a true sadist. Together we are very very extreme. He really gets turned on by really hurting me and seeing me trying to struggle and get free. Outside of this role, he is the salt of the earth, he is like 2 different people and would would never believe he was capable of doing the stuff he does. I know this is different because its how we like our relationship, but I think I'm trying to say that you can't judge someone just by the personality they are showing you. I think of personal safety all the time.