I've had a hemangioma in my masseter muscle for the last ten years. It hasn't grown, but should it be removed? It doesn't hurt but I am aware of it.

It depends. How do you know it is a hemangioma? Only a biopsy can confirm such a diagnosis. Imaging studies can suggest a hemangioma but it is not definitive!
If it ain't broke... You need your masseter muscle for chewing so it would be a big deal to 'cut it out' . It appears the 'benign hemangioma' is stable so why do anything? The risk outweighs benefit here 'if it ain't broke...'.