A common misconception is that the pH of your water is important in all grain brewing. Actually, it is the pH of the mash that matters most. The water parameter that you should pay the most attention to is the alkalinity (or more appropriately, the residual alkalinity) because the pH of the mash is largely determined by how the acidity of the grains is balanced by the amount of alkalinity in the water. For example, if you brew a dark beer like a stout that has a lot of very acidic, dark roasted grains, you need water with high residual alkalinity to balance that acidity in the mash.
The reason many brewers sometimes dilute alkaline water is that they will be brewing with very light grains, which contribute little acidity to the mash. Unless their tap water were diluted, the high alkalinity in that water could cause the mash pH to be to high for proper enzymatic activity, and possibly lead to pH problems in the boil which affects a number of flavours (e.g., hop bitterness).
The pH of your water is, unfortunately, not a good indicator of its residual alkalinity. So, if you want to figure out your mash pH, you will need to use some basic chemistry to determine how your water will react with the grainbill, or you can use a software tool to do it for you. Ken Schwarz's BreWater software is a great free example. John Palmer's water nomograph is another great tool (essentially a graphical tool).
Or you could just dilute down your tap water a bit to get it in a reasonable range, and then toss in some pH stabilizer, like Five Star's 5.2 pH buffer, and not worry about all the messy details.