I'm also looking into Mirena, i haven't gotten it yet. I have read however that if the string is further down/up, check with your doctor just in case (I'm assuming you got it recently). If it IS slipping you don't want it to slip out/get stuck, in *some* cases this can happen and may need to get surgically removed in extreme cases. Better safe than sorry!
no they never told me to check them i had mines in for 4 yrs