I've never deliberately posed as the opposite gender; I just don't specify what my gender actually is when I'm online, and I don't answer when people ask me. I don't think it's particularly important.
If it's not important, then why don't I just specify? I'm a fairly androgynous individual in my personality, and I prefer people don't make assumptions about me without bothering to get to know me first based off a gender sign. I also find it quite interesting to see what people assume about me when they don't have that to go on. Ask people what they think of you and they'll usually either lie, tell you what you think you want to hear, or give you an incomplete picture because they don't want to offend you. Regardless, you never get a satisfactory answer...or at least, I don't.
But I've actually learned quite a bit about how people see me and how I subconsciously portray myself by what gender people assume I am, and this sort of thing interests me. I've told some people after a while what my gender is, or they've found out through Skype calls - bit hard to disguise that sort of thing really - but then I've never made a big secret out of it or anything; as I said, it doesn't matter, I just prefer to not specify if it doesn't inconvenience me to do so. I've not told anyone on here yet, though. I expect I will eventually, but I'll never make it public.
So, it doesn't matter to me what gender I am and what gender people think I am, but it can be quite insightful. It doesn't matter to me what gender other people are, as I'll treat people the same regardless and try not to make those initial assumptions (not sure how successful I am with that, but I try not to cast things in stone) so it's never bothered me when I've gotten someone's gender wrong...unless I address them and they correct me; that's inevitably awkward, aha.