It is tempting to be all the things you wish you were but aren't in person with your on-line persona because it is likely that no one will ever know you are being dishonest. However, you still know you are lying which should be enough to keep you honest.
I give slightly more thought to a written post than I would when conversing with someone in person. With respect to that I am less apt to suffer the foot in mouth syndrome, but not always because I still screw up what I want to say.
It is next to impossible to know how what you post will be received. Some will read an attitude into your post when you don't have one. In contrast, it is easier to conceal your real feelings when you write as opposed to when you speak.
Considering all of the above, I try to be as much myself as possible when posting. I deplore dishonesty in others as well as in myself.
