If you loosen the set screw your tranny will completely lock up and you wont be able to roll backwards at all, and in worst case, wont be able to move forward either.
If your engines are broken in for some time now and you still run into problems shifting, then I definately suggest adjusting the shift point to allow it to shift sooner.
When you adjust your shift point, do it in 8th turn increments and run for a few minutes. You dont always see the effect immediately.
Also, keep in mind, the leaner the engine runs the higher the RPM's are achieved. If you are taking temps you will clearly know if you have room to go on leaning the mixture.
I adjusted both mine and Woodies with the HSN and never touched the tranny. Both of us were running around 300 when we got there. Mine was 280 and his was about 275.