This can also depend upon skill level. Rob Leatham calls his shots almost exclusively based on the sights before he presses the trigger.
Lately, I have been calling my shots based on a combination of the sights/dot before I press the trigger, how the trigger feels during the press, and anything unusual I see in the dot during the press. A red dot pistol is very helpful to me in developing shot calling ability, regardless of whether you shoot a dot or irons.