i did test ..rendering out 4 generations ( using contrast/brightness fx) i could NOT see any difference between rendering using GOOD or BEST ... viewed on sony PVM 14M4U monitor ..
The difference will primarily be seen in downsizing of source footage (especially reduction by greater than 50%).. For example, if you are using 720x480 source (like DV) and saving to 320x240, it will _sometimes_ yield better results. However, if you are using a lot of BIG still images like 2048x1536 (3.3 Megapixel cameras) then the resize to 320x240 will likely look better
Best is slower than Good though... and not all source footage will look better when rendered as Best. But it's there if you need it.