I'm pretty sure that if you had a salt build up your run off PH will be lower than desirable. Your run off PH is indicating that you haven't got any salt build up.
Personally I don't flush, maybe only before flowering. The reason for that is because of the plain watering's in between the feedings. That usually keeps any salt build up from happening.
Have you got a TDS/EC Meter? Measure the TDS of the runoff, if the TDS is way higher than the PPM that you've been feeding with then it may be time for a flush