well in general terms....with lying you have to remember every single you thing you have said to that person to keep the lies going and that can be draining and stressful. it also makes it difficult if you are lying about something and then something actually happens and you really need a friend to talk to but you are unable to because of your lies. telling the truth is less stressful and you can talk about anything without having to figure out what u have said before then.
honesty is important in a D/s relationship. if you aren't honest with your Dom/me and they do something it can be painful or it can injury you. Being a submissive I tend to rat myself out even if I don't mean to ..it is easier than having to tell them later.