So basically we are flying guinea pigs.
In a manner of speaking, we ARE all guinea pigs - as have been our ancestors and precursors in evolution for thousands of millions of years.
The earth has a cosy skin of atmosphere and a powerfully protective magnetosphere to somewhat dilute the quantity and intensity of incoming ionising radiation. Cruising above a portion of that protection in the stratosphere definitely increases exposure, but the effect is not simple to intuit or measure. Depends on age, gender, diet, genetics and probably a fair amount of luck whether a particular cosmic particle will tear up a particular piece of DNA that will replicate and cause a particular cancer -- or not. Our DNA has learned from all that historical radiation to be very good at repairing itself - and at killing off improperly broken cells, so mama nature helps to mind the store even when we are in zipping around in our spaceships.
Pilots are very easy to identify and track for medical history purposes. So far there seems to be no persuasively documented 'smoking gun' of radiation-related diagnoses in fliers. This contrasts sharply with some workplace chemical exposures that leave an unmistakable trail of illness.
Is good to be cautious, but probably, as Dani says, the fatigue and stress connected with a flying career will get you - if not managed skillfully - long before the glow-in-the-dark stuff.