Is there visual evidence in any of the original three films that deflector shields do anything on any small fighter craft? There are plenty of scenes with the Millennium Falcon or Star Destroyers shrugging off weapon fire with a brief flash from the shields but it seems every time they show a fighter getting hit, it destroys the vehicle (except when R2 gets hit).
In Star Wars: A New Hope, Wedge's X-Wing takes a direct hit to the rear quarter. Some of the energy seems to have been taken by his shields
This ties up nicely with the description in the film's junior novelisation. Evidently the blast damaged his controls and shield generator.
The three TIE fighters swooped into the trench. Luke focused on his targeting scope, which had just marked off the distance to the target. The TIE fighters zoomed closer to the X-wings. Darth Vader fired.
“I’m hit!” Wedge shouted as his ship was blasted from behind. Although his ship was still intact, his deflector shields were lost. Realizing he wouldn’t stand a chance against another attack, he said, “I can’t stay with you.”
I think the real answer is "it depends". On what? Dramatic license.
It's not uncommon in such films that the hero's ship can take several hits, yet destroy its opponent with a single shot. This is done to build up tension.
Similar to the example above, Star Trek has shown ships being physically damaged, before cutting to a crew-member claiming that the shields are down to 30% (or other arbitrary number).
In most cases, the writer has some idea of a shields/forcefields effectiveness, but will succumb to dramatic license as needed.