Pre-filled saline syringes and checking central line blood return

Elsewhere on allnurses.com, a poster pointed out that one of the reasons why nurses shouldn't use prefilled normal saline syringes for diluting IV medications was that, in many cases, only the saline and interior of the syringe is sterile-- if the plunger isn't sterile, when you push out saline to make room for the medication, microbes on it can contaminate the barrel, and when you pull up the med, the diluted medicine contacts the contaminated barrel.

Doesn't the same thing happen when we use pre-filled syringes to check blood return on a central line? We're taught to flush the line with a couple of ml's of saline (which could theoretically contaminate the inside of the barrel), then pull back to aspirate blood and verify blood return (which brings the sterile saline into contact with the contaminated barrel) then continue to flush.

BTW, our sterile syringes are packaged in plastic labeled "DO NOT PLACE SYRINGES IN STERILE FIELD", which suggests to me that after the contents are sterilized, the syringe is processed in a non-sterile environment, so the plunger shaft is non-sterile.

Should I be paranoid about flushing with pre-filled syringes?