Rearfacing to the upper most limit on your car seat is recommended. Rear facing is safest for any occupant for that matter. With that said if you feel your little one may be too scrunched or uncomfortable, you may be able to give them a bit of extra leg room by reclining the seat they are facing. Our little ones rear face from the beginning. They do not know any different until you turn them around. 12 months AND 1 year is the minimum to rear face.
If you have a feeling that your child isn't comfortable in their seat, it may just be a matter of the way in which the car seat is installed, or the way in which the child is seated/positioned in it. I would recommend visiting a certified child passenger tech to determine if there is any misuse which could be contributing to positional or other difficulties. www.seatcheck.org