Simple answer: Not really, no. Especially for a beer like a porter (or anything else darker than about 10-15 SRM). I do manage my sparge water for something like a Kolsch or a Hefe. But, you are unlikely to have any issues with something as dark as a porter. For this to be a concern with a porter, your sparge water would need to be 150 ppm CaCO3 or greater.
If your water has that much buffering capacity, then I would just cut it with 50% distilled water.
Detailed answer: I'm a firm believer in the "measure and then adjust" approach. Acid is an adjustment tool that should only be used based on a measurement. If you are dead set on acidifying you sparge water, add a few drops of acid (lactic or phosphoric) to bring the pH below 6.0. Then monitor the pH of your runnings, and adjust as necessary. Both acids can have flavor impact (good and bad), so I do try and minimize their usage. If I can accomplish my pH goals without acid, that is preferable.