With flaps 15 and both engines at maximum take-off power, the aeroplane should be held on or near the ground until the take-off safety speed of 97mph is attained.
I have a theory about this one.
The PA30 was involved in a number of training spin/stall accidents in its early days when the FAA still required a Vmca demonstration in checkrides. The problem was, in essence, that the rudder remained effective down to speeds very close to stall, and the Vmca demo rapidly turned into an asymmetric stall.
The "fix" was straightforward: the FAA simply increased the nominal Vmca, by a fair few knots. As a result, the take-off safety speed increased proportionately, but, I hypothesise, the instruction in the POH remained the same with the new figure inserted. Perhaps it was test flown a couple of times on a nice day.
As a result, the instruction is, IMHO, dangerous in many conditions. The high nose attitude on three wheels of the PA30 means that at 97 mph with flap 15 you're either climbing or you're wheelbarrowing. The unofficial Owner's Handbook for the PA30B (well worth getting a copy, Essco sell them) quotes take-off runs based on a lift off speed of 80 mph with flap 15, with an assumed speed of 91 mph at 50 ft.
I don't use flap for take-off, though I don't tend to operate off short runways. I lift off when the aircraft wants to lift, and accelerate to about 90 kt before tucking the gear away and climbing.