In my PICU, we draw labs off of PIV's if we can get them. We get a fair number of DKA kids, and they always get two IV's. One is for their meds, fluids, and insulin drip. The other we save for blood draws. We are draw glucose levels and blood gases every hour on these kids and BMP, Mh Phos q 4. I can't imagine finger pricking or sticking a kid (or an adult) every hour for labs, so we use the PIV. We don't draw from the line for cultures (obviously) or for some labs that might give us false readings, depending on the IV fluid running through.
I do find, personally, that PIVs are more likely to become infiltrated or clotted if used for blood draws. I think it's due to manipulating the IV in the vein. I'll still try it, though, especially with a child who is a hard stick. Best case senario is I get the blood and keep the IV. Worst case, I have to start a new IV, which means another needle stick, but the child would have needed another stick for the labs anyway.
Central lines are larger and sit in a much bigger vein, so it's far less likely that drawing from a central line will cause a a problem with the line. The blood in the central line vein is also diluted more quickly than a peripheral (think larger vein, more blood, faster blood flow), so it's less likely that a central line draw will give you false results due to fluid running through the line.
We don't ever return waste, unless it's a kid that needs all the blood he can get and we have a specific doctor's order. And then I'll always get another RN to gently shake the blood in the waste syringe while I draw the labs. I've seen how quickly blood can clot in a syringe, and I can see how dangerous it would be to return blood that could be full of micro-clots, not to mention the greater risk of infection.