When I worked in chronics, we would not attach a syringe to the fistula needles; the heparin would be given through the venous line, and the arterial would simply be allowed to "flash back"; just prior to attaching the arterial blood line, we would prime the arterial fistula needle with blood (by twisting the end cap). The drawback of this approach (not having a syringe attached to the arterial fistula needle) was that you could not always tell if you had a good stick until you got high arterial pressures; then you still had to get a needle and adjust it while you already had the blood in the system. But this was company P&P and we had to adhere to it.
Now in acutes, however, we attach syringes to both fistula needles; after cannulating, we pull back and flush blood (have to be careful not to infuse air! But we're all RNs). I think when I'm on my own (and not under constant scrutiny during my orientation), I'll have me a couple of saline syringes ready for the task (a blood/saline solution is much less likely to clot while you're getting started).
As for the heparin, to my astonishment acute (hospital) nurses are much less worried about (a) giving it at all - we have a lot of discretion, and (b) giving it right at initiation of tx, much less waiting 5 minutes to start. Interesting how the approaches differ