The issue is not as to whether the wort pH comes in too high or low but as to whether the runoff, at any time, reaches a pH higher than the high 5's in which case tannins may be extracted from grain husks. Clearly if you collect 19.9 gallons of wort before pH 6 is reached and the last 0.1 gal at pH >6 then you are not bringing much tannin into the beer. OTOH if you bust 6.0 when only half the wort has been collected (10 gal) you could be in trouble in this regard. It is also pretty clear that if you acidify the sparge water to 5.8 before the commencement of sparging you will not be busting pH 6.
It is best to monitor runoff pH during sparge now and then to get an idea of how long your runoff stays < 6 without treatment and base your decision to add acid or not on that.