I have both 24 and 36 inch rails. Actually 2 sets of 24 and 1 set of 36. I can count the # of times I needed the 36 inch on 1 hand. I never use them now as I rip down wider logs with the mini mill. To me as a professional furniture builder, Boards over 15 inches wide are useless. They warp, twist, crack, and simply become narrower boards. I can mill 22 inch logs with a 24 inch mill. The usable bar length is 18, but by rolling and slabbing the log, you are down to 15 to 17 inches in no time. In my experience you get better quality boards this way.
The 2nd thing is, from a safety standpoint, having 12 or more inches of unused bar extending out the log is a disaster waiting to happen. Slip and lose your footing, and your arm is gone. I compare it to using a table saw. If your ripping 1 inch thick boards, would you set the blade height to 3 inches? That would be unsafe, we all know that.
Just an observation from milling, and liking all my limbs. I always run a 24 inch bar if possible, from a safety standpoint 1st, and 2nd, running a shorter bar is always easier on the saw. I have run a 36 inch bar on my P51, but its a totally different animal with a 24 inch bar and low pro chain.