In order to determine if you actually need a flush in soil, start by testing your PH.
1. Test your PH, before you water.
2. Water until you have 10-20% runoff.
3. Test the runoff PH.
4. If the difference is more than a half of a point, consider a flush. If it is more than 1 full point off, a flush could be very helpful.
So if you put PH 6.0 water in and get PH 5.2 out - you probably need a flush, but if the runoff is PH5.6 or close to what went in, you can wait until your last few weeks of flower. If it comes back at PH 4.8, I would flush, as your buildup is too high.
The final flush 2 weeks before harvest is the most important, to rid nutrient build up and prevent it from harsh-ing up your final buds.