Yes, pretty much all chargers will operate most efficiently if the headlight is turned off while charging so maximum voltage and current can be applied to charging. Bicycle dynamos are very useful, but really don't produce a lot of power at this time (there are a few three-phase models in very limited production but not widely available or packaged so nicely as the SON or Shimano hubs) and devices are getting more and more power-hungry. Some phones have a "trigger voltage" minimum that must be met before they will even accept a charge, so it pays to match your devices and charging source with care to get the best results.
I view effective, efficient charging while on-tour as a lifestyle. It is a lot to remember at first but soon becomes second nature. My preferred tours are most often in backcountry where there is no access to mains power and it is helpful to have a way of producing your own. However, if you are forced to travel slowly you will find your dynamo charges slowly as well. Many people get by with large-capacity rechargeable piles (batteries) and I certainly did so happily on my European double-crossing, where I could recharge it from mains power at least once a week. I combined this with a solar panel that had its own accumulator battery as well. Others prefer to charge their storage batteries from their dynamos and then use that battery to recharge their devices (expect about a 1/3 loss in charging efficiency).
There's a number of options, but for easy, largely consistent charging while under way, a dynamo is hard to beat and my preference is for one located in the hub. If you don't ride so much each day or ride slowly and are near mains power once in awhile, there may be better options for you.
Best, Dan.
