i'd see if your nutrient brand makes a suggestion. e.g. mine says 6.0-6.5 and it is a hydroponic fertilizer. if i go below 6, i have problems. so, there are too many variables to follow some blanket suggestion, but it can get you in the ballpark. you want to try that low? if/when a problem shows up or you don't get through an entire grow without minor issues requiring supplements and other issues that just don't seem to go away, be willing to accept the pH is too low.
plagron will have a suggestion, i guarantee. whatever it is, do what they say for their nutrients. they know better.