Re: OCW method
Yes, 100 yards is ideal for most cartridges as consistency of POA is vital with OCW testing. Some magnum cartridges may require a standoff of 200 yards as they may not group well at 100 yards because of stabilization issues.
With Ladder testing it's a bit different - you must place the target far enough away so that differences in velocity from load to load become apparent from vertical stringing. This technique is a bit more lenient on POA compared to OCW.
Both have pros and cons however.