Yeah there are an awful lot of different options and methods but here's how i've been taught here on this first grow:
when I make a fresh res I make two instead of one - both identical. If the EC is in the correct place (it will be if you're a good grower) then this second res can be used to top up the main res. Obviously it's almost impossible to find out which nutes the plant is using more than others but if you were overfeeding I'm sure you would see signs of toxicity, because both res's are identical its more like topping up the level than the amount of nutes - does that make sense? I Imagine if the Ec is correct then the plant will use the nutes in the res evenly.
Then we come to the plain water feed. This is only used if the EC has increased over a period of time because it will set the res to a lower EC from which it wont rise.
so to summarise:
The identical back up res won't increase the EC, it just keep the water level topped up.
The plain water feed is used occasionally to lower the EC if there has been a rise (means the EC was set too high - use water to take it lower)
I'm really bad at explaining things so please feel free to ask away I'm happy to (try and) help