I had flat nipples and didn't realize this until DS was getting frustrated at the breast when we first tried bf at the hospital. A nurse gave me Nipple SHELLS, which is way different that Nipple Shields. You wear the shells prior to nursing and it helps to draw the nipple out for the baby to have better latch. You take the shells off and baby latches on to your nipple that has been pulled out. I don't have to wear them anymore because bf for 3 weeks has helped pull my nipple out enough that latching is no longer an issue.
Every BF book will tell you to AVOID the use of a SHIELD. You wear the shield WHILE you nurse and this can create problems because the baby will get used to latching on to the shield and not your nipple.
If you are worried about having flat/inverted nipples. It is safe to start wearing the shells for half hour a day in your 3rd tri to help draw your nipples out.
good luck