Bass Guitar Open String Notes

How to take C / E or F / etc and the guitar?
I understand that you change the chord normal, taking another low note. If, for example, you wanted to have a F / E you would play only 1-1-2-2-xx Since then, the rope 2 in D will become an e. But why is this more preffered playing just 1-1-2-3-3-0 (which is playing the string open top so as to obtain the low and how)? There is a rule for this? In addition, the A / E that you would play down the rope and open (0-1-1-1-0-0) instead of just playing the lower four strings (which is 0-1-1-1-xx)? I was looking for a theoretical general rule here, but it seems that your answer is "yes" I may have the chords for the two sides, while the top string that you play the bass in question?
Why Cadd9 against G a prescription or open, when some can not tell the difference, and some may very well ...? Reason, there are nuances such as the ear can pick up, especially when combined with the human voice. These notes varied bass strings are just a different icing for a cake differently.
How To Play Bass Guitar - Lessons for Beginners - Open Strings