The main reason for backing off the rocker shafts is to save your springs. Having a 500# spring compressed for 6 months is probably not that great for it...it makes sense. A secondary benefit is it seals up the motor.
To the OP, Just back the shafts off slowly and evenly, then put them back on the same way...slowly and evenly (1/4 to 1/2 a turn at a time). This is the only way I have ever put mine on...I don't think I could get the push rods in after installing the rocker shafts.....well, I could...but it would be a PAIN in the 8 3/4....:D