For 'S-n-R' - I have never been skilled enough to notice discernable handling changes on the 737 with CG movement. Pax distribution is pretty crucial, and model specific, and you should fairly quickly be able to pick up a pattern of how/where to load pax and bags for each a/c. My basic guideline (7372/3/4/5/7) with equal A/B/C pax is 100 in 3/4 and rest in 2 (very small trim change in 2). I don't fly 8/9 but I believe the pax need to be biased forwards.
The loading guidelines we have from various airlines for 737-300/400/700 (never worked with -200 and -500) are very similar to what you describe, normally fill up at the rear with 100-120 bags, then overspill into 2, with some minor variations.
The 737-800 is a different case and a curious one from my point of view. There are two major tendencies followed by the airlines we operate with: some say 2/3 in hold 3, 1/3 in 2 - using actual baggage weights - and some always require first 140 in bags hold 3, rest in 2 - using standard weights of 13 kg per bag. From my own experience at check-in, I know that the actual bag weight can average 17 or 18 kg most of the time on said flights. Now let's suppose you have a 737-800 with 140 pax seated evenly across cabins, and that those pax have checked in 140 bags. As per standard loading, those bags are loaded into hold 3, and the aircraft is well within CG limits. The loadsheet will show those bags as 1820 kg due to the standard weights used, but in reality they will most likely weigh around 2400-2500 kg. My question is, and particularly considering the low ground clearance of the -800, can this scenario pose a risk or is it not significant enough?
Cheers