A little more info might help with an answer. Why are you looking to dilute the wort? Are you brewing extract and doing partial boils? If I had the choice, I would tend to want to add the water pre-boil, if only to make sure the water addition was sanitized. You can, of course, boil, cool and add the water at any point in the process, I don't know if it would make any difference.
In addition though, I've read that the volume of boil water affects the hop utilization. Less water equals lower IBUs. It may have an affect on an IPA. My brews are very low IBUs anyway, so I can't say from experience.