In a 1-and-1 catheter/cannulation procedure, which one should the needle be? Arterial or Venous?

For years, I have always done it as venous at slow flow to help "balloon" the access. I'm told, though, the needle should be placed at the potential venous area, but as an arterial pull initially.

Why is that?

The proper procedure is to return the blood via the catheter and pull from the fistula.This will prevent infiltrations and it does not increase the size of the Fistula .Go slow and the Fistula will last ...

The 1/1 technique is not used to help "balloon" or mature the access, it is to assure that cannulation can be performed safely and without damaging the fistula. Fistulas which aren't mature enough for cannulation will infiltrate ("blow") very easily. An "arterial" infiltration causes much less damage than a "venous". Keep in mind that with venous, if the fistula infiltrates, the machine forces blood into the surrounding tissues.
This is an often misunderstood concept. I often hear nephrologists tell the staff to start fistula use by returning through the fistula. As this can be construed as a medical order to be followed as such, we as nurses need to keep in mind that we are obligated to clarify any gross errors with the prescribing physician.

Old School is right.