I used a shield for the whole time I BF my dd which was about 5 months (4.5 months full time, half a month part time due to returning to work).

If it is working and your bub is gaining weight then I don't see a problem with it. You can try now and then without the shield, even if you get bub to start on the shield and suck for a while to pull the nipples out, then take the shield off (messy and not easy). I think I only had success without the shield twice.