I am a big fan of decline benching, as it tends to take the shoulders out of the movement a little. As for flyes when trying to add mass, I see no problem with them. I do them at the end of my workout to get a good stretch and to get that "pump."
yeah i always do flyes on a flat bench or incline bench at end of workout for a nice stretch you can really feel those, just 3 nice sets, same weight for around 10 reps.