Is it safe to travel by helicopter to another hospital if having a stroke?

Most of the time. Weather conditions are quite important. The speed gained by helicopter transport can be important in improving the odds that urgent treatment can be effective.
Yes. There is a movement in the usa to have specialized stroke centers and, in particular, if the stroke victim is to be treated with a thrombolytic ("clot buster") drug, they must be in a hospital with neurosurgical backup. Helicopter is the fastest, safest way to go. (also the most expensive, unfortunately).
Yes. Medical air transport to a stroke center is generally worth the risk if it means receiving better emergency care.