I think you are mistaking the effect of weapon rarity. Based on my analysis, rarity increases the number of mods a weapon may have. But it does not affect the weapon's type or damage. You can get plenty of Epic weapons with 3 useless upgrades (reduced recoil, high air resistance, etc.) and then get a Rare weapon with 2 good upgrades (120% damage, double number of shots).
So, while the chances are higher that an epic weapon will be good, the advantage isn't significant enough to warrant skipping rares.