I prefilled 12 of my syringes and I was just fine. But I do believe that the plastic absorbs some of the hcg so it could be that I didn't need as high of a dose as I thought....
mikkydee's sight is www.gphcg.com He doesn't recommend prefilling syringes for more than a few days at a time due to the plastic absorbing the hcg. Also you have to shield the hcg from light so if you do prefill wrap the syringes in something to keep light out.