Both my children had plagio, and were treated in different ways. With my DS we used repositioning, as he was a good, still sleeper, and it was easy to ensure he used all parts of his head, coupled with being in a sling most of the time.
DD had helmet therapy, as she never slept for more than 30 minutes (reflux) and didn't keep still. She also had a sling, but she sat quite early, so played sitting up, which made a big difference.
I used a goigoi pillow (swedish I think) when they were in the pram, as I'd be able to observe them all the time. Never used it when they were in the cot as I couldn't be sure there's be no risks- that said all babies in Sweden have pillows, so I suppose it's a cultural thing, I believe their rates of cot death are very low.
You do need to have it checked- as others have said, she may have torticollis which is why she prefers one side. We had physio exercises to do, and we just did them every nappy change, and it really improved.
There is another condition (that I cannot recall the name of, I'm sorry) where the plates of the skull fuse too early, so you do need to get your GP to look her over/refer to Paed, but it's very, very rare.