My 4yo has always chewed on one side of his mouth, though in recent months has become more cooperative about sometimes chewing on the other side. But his teeth often squeak while he chews now, especially when he chews on the 'old' side, IYSWIM.
His jaw has always been lopsided - it seems perfectly symetrical from the outside, but the bite was very crooked from the inside, with only one side meeting. I think that's why he got into the habit of only chewing on one side. But, oddly, he chews on the side that doesn't seem to meet. As he grows, it is becoming more symmetrical.
Do I need to worry? So far, whenever he has gone to the dentist, they have said that it is far too early to be concerned by anything structural in his mouth, including his finger-sucking. But the last time we wnet to the dentist (it was only the first time this dentist had seen ds) she said that we needed to wean him off finger-sucking as it was making his jaw crooked. But I know that his jaw is actually straightening out! OK, his front two top teeth are a bit crooked, but the rest is straightening but she hasn't seen him before so she doesn't know that.