David, don't worry, the flights across the North Atlantic are some of the least troubled by turbulence - which is not to say I have never experienced it of course, but I can't remember the last time.
Generally areas near the Poles are much better than areas near the Equator. I've also heard about running into stuff across the Pacific as well (but possibly it's on Australia flights across the Equator).
If you're coming from the West Coast there can sometimes be a bit across the Rockies but that's it for predictability.
It's the same when I go from London through Hong Kong to Australia - always nice across Siberia, any unsteady part comes in the Tropics across Indonesia and Northern Australia.