I PH the RO for 3 days or so and then don't bother for the rest of it. You're only adjusting PH to allow the plant to take in nutrients, there's sense to that for a few days whilst there's still some in the medium but after that there shouldn't be anything left to take up, therefor no need to adjust it.
At least that's my thinking... it's a hot topic so I'm sure someone will have a different opinion. Always worked for me though.
If you're flushing with tap, it may actually be better not to PH it, as you don't want the plant taking in the additives within the tap water, ie calcium, magnesium etc.