There are certain features which make a sacral dimple more concerning and mean it should be checked out (eg if it is large, or hairy, too deep to see the bottom, high above the anus, has a birthmark etc). Without those features it’s probably fine.
I seem to recall that ultrasound can be used to detect spinal issues when they are newborns. When they are older it needs MRI as ultrasound can’t get a good enough look by then. Ultrasound is a lot cheaper and easier to get than an MRI, therefore if you have any concerns, it’s worth getting it checked out now rather than wait and see.