For your extract brewing, the most important thing to control is alkalinity. You do need to acidify the water prior to steeping so that you don't extract tannins from the steeped grains. In addition, you will want to acidify the dilution water used with your extract since high alkalinity water will raise the pH of your finished wort and that can make the hop flavor 'rough' and higher pH beer doesn't taste as crisp and refreshing as beer that achieves a proper pH. If the wort pH is high, that can elevated the resulting beer pH.
Bru'n Water has the tools needed for calculating your acid additions.