My professional advice for you would is to sit down and compare different settings until you find what sounds best. Setting the crossover is more of a personal prefference. I usually have mine set around 80hz, but when listening to some older rock I will bring it all the way up to 150 on certain tracks. Variables such as your system's components, enclosure, the music, and individual hearing will all contribute to the crossover setting. Although in short, yes, 70hz is a common setting.
I like mine around 100ish, like zac said... all about personal preference basically... change it around and listen to the type of songs you'll be listening to on it mostly, then decide what sounds best.