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

Yes: Medical air transport to a stroke center is generally worth the risk if it means receiving better emergency care.

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).

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.