curing them releases anything they will leach, by not curing them you can have a spike in ph from the calcium contained in the concrete if i'm not mistaken,
with that said i have used way more than 10 at one time and have not had any problems, however i have a large water volume so if you have a smaller water volume i wouldn't use many at once,
the left over plugs you have you can put in a ziplock bag full of holes in the back tank of the toilet, they will be fully cured in a week or two.