For DH it's pretty much about BB height and ground clearance when pedalling, as stated by others.
What sort of frame do you have? If you have, for example a Sunday, which has a very low BB height, a bit over 13", I would definately go 165mm. If you have over 14", you could go 170, but IMO the default is 165, and you should come up with a good reason to use anything different.