I treat anything under 4. That isn't a "true hypo" as non-diabetics can get lower and be fine. It is just the lowest number I'm willing to see. It matches the guidelines that my doctors, nurses and members here would suggest.
If I have IOB and test in the 4-5 range I'll either put a minus temp basal on or eat something, sometimes even both. I do that more by instinct than hard and fast rules.
At the end of last year I stopped feeling hypos as sure as I had. I even tested in the 2-3 range a lot of times without feeling it.
I raised my target BG by myself. The next visit to the nurse I told her about it and she suggested I up my target even more. I'm talking 7 during the day and 10+ overnight.
That seems overboard but running that high isn't going to kill you overnight, but a hypo might.
I've been lucky and stopping almost all hypos for a couple months has lead to me getting clear warnings in the 3-4 range again.
I've now lowered my targets again. Not quiet as tight as before but hopefully the right range to be safe and to get a 'good" Hb1Acs next set of bloods.
I hope my post gives some answers on my thoughts on the matter.