3 days

if you start out fermenting at 64 F and are not seeing robust activity...could you wait until the 4rth day and raise it to 70 F...
by then your risk of off flavors will be somewhat decreased??
as opposed to not waiting 3-4 days...??

It depends greatly upon the strain of yeast used, the health and age of the yeast, and the gravity of the wort.  If you are seeing any type of activity, I would not be concerned.

If 64F is at the low end of the manufacturer's recommended range for that yeast, I would slowly ramp it up after 48 hours.

If it's working then it's fine. What you have to worry about is unwelcome stuff growing in it, but as long as there is yeast activity then it will crowd out any bad stuff that might be in there.  I've had some ales take four or five days to really take off, while others are done in that same period of time.  Remember that you work on the yeast's schedule, not the other way around.