To be honest, I would only go on at night too when I first joined... but having been here a while the whole idea of what I represent myself as, as a member here has made me more relaxed about it all, especially with being on the forums with parents around.
Still, if my mum walks in i'll switch tab. If she catches a glimpse of it she'll think it's just a "silly chat room internet thingy" so I don't really bother about it too much. I don't clear my history, I don't log out from the site... cos my mum is as computer literate as a giraffe - she wouldn't have a clue how to find out anyway.
And like Lukie, I have my own room and own laptop, whereas my mum uses my desktop for all her crap. When I'm not here (i.e. at uni) my laptop is with me. Really my only problem is that privacy is not a word in my mother's dictionary... unless the lady is in, then it magically appears ¬_¬
But seriously, if you try to hide it, you risk showing that your hiding something, which makes parents curious, which leads to them looking through all your stuff to see what your up to. My advice really is go on whenever... just be swift and cool about switching tabs or windows or something and they won't suspect a thing ^^,