I don't think water should be adjusted for extract brewing because the malt maker has already done that when he mashed the grain in the malt extract-making process, presumably adjusting the water to suit the color of the extract. Stripping the water out of the wort to make extract will leave the minerals and salts in the extract, so they will be in the beer when you make it. Any ions that are precipitated and thus removed by the mashing process don't need to be replaced, they've done their job.
That being said, If you have soft water, no chlorine/chloramine or other off tastes in your brewing water, leave it alone unless you are making a beer that's at the extremes of the malty-hoppy balance. Then you might add some chloride or sulfate to enhance the flavors you're brewing for.
If you have very hard water, I would either dilute with RO water or find a source of softer water. I do think that too much bicarbonate will give some harsh flavors to light colored/delicate flavored beers.
If you don't know what minerals and ions are in your brewing water, find out, so you have a starting point if you do want to tinker with the water to enhance the flavor of the final beer. I would suggest you use the final volume of the beer as the basis of any adjustments you make.
