Generally, you would want to trim them after they started growing in the spring. This will allow you to see if last year's growth survived the winter. Plants that are cut back every spring should grow into a starburst pattern with long arching stems. If they are not cut back, much of the growth will occur from the previous year's stems.
Southern Missouri represents the northern edge of beautyberry's natural range. Winter temperatures are the main factor in survival from year to year. It would be good practice to mulch them heavily each fall; piling about a foot of leaves around them will help insulate them. The excessive leaves/mulch should be removed in March or April.