Dean,
We have done client 6.5.6 installs, and none of them have failed, but none of them have been a turnkey walk in the park.
If you are going forward (having already validated that everything has been brought current and to spec), I'd suggest a Thursday night implementation. If things go sideways you only have one patient day before the weekend.
If there are things that need tweaked that come up on Friday, you have all weekend to tweak.
