I play viola, I don't hold it in the full classical position, but in a more relaxed way. I don't have any trouble shifting between 1st 3rd and 5th positions.

It's most important to find a way of holding the fiddle that enables you to do what you want whilst being comfortable. If you put strain on your body. you probably won't be playing for very long. People vary - length of neck, slope of shoulders etc - what works for one person may not work for another.