Do you carry-on if possible?

I'm flying UC SFO->LHR on Wednesday. I just completed a "trial-pack" and discovered my wife and I can get by with a mere 18 pounds each in a US carry-on approprietly-sized bag (the AirBoss from http://www.redoxx.com). For a 2 1/2 week trip at that.
We weren't intending on carrying on our main bags. It didn't seem right in UC, especially as the bags should come off early in Heathrow.
Am I crazy to not always try to carry on? Since this is a direct flight, I don't think we'll have trouble loosing our bags, but I may be wrong (or foolishly optimistic). Any benefit at LHR in terms of getting through customs more quickly if you're not waiting for your bag, etc.? Or should I just relax and let Virgin take care of it all.
We weren't intending on carrying on our main bags. It didn't seem right in UC, especially as the bags should come off early in Heathrow.
Am I crazy to not always try to carry on? Since this is a direct flight, I don't think we'll have trouble loosing our bags, but I may be wrong (or foolishly optimistic). Any benefit at LHR in terms of getting through customs more quickly if you're not waiting for your bag, etc.? Or should I just relax and let Virgin take care of it all.