Incline DB curls (head remaining on the incline board throughout) allows a better range of tension (not to be confused with ROM) than the standing version of DB curls. With the DB Inclines, start with the hammer position at the bottom position. As you curl up, twist the DB's out so the thumbs will be facing away from the body at the top position. Give a slight crimp to the bicep and lower with a reverse of the twisting motion. Twisting the hands/wrist/forearms has a strong influence on building the biceps. That is their natural muscle curve/function. Gymnast who twist the hands a lot (Example...the rings) seem to have pronounced biceps.
The anterior delts (front) will be called upon, to a lesser degree, when doing most regular DB/BB standing or sitting curls. With the angle of an incline bench, the upper body is supported and the arm's are hanging somewhat freely by themselves when doing DB curls. There should be very, very little influence on the anterior delts. If stress is felt on the delts it may be due to the stretch from the weight of the DB's in each hand. Or a person is just doing the exercise totally wrong. Good Luck.