Generally the tighter the better (up to a point). Tight enough to significantly raise the upper wing of the ff. (Somewhat counter-intuitive.) Getting the G to sound good is 7/8ths of the process. This is accomplished with lateral adjustments of the post (tightening). The sound quality produced by the G, and the general physical responsiveness of the instrument are effected by these changes in lateral position.
The remaining 1/8th (the remaining 3 strings) are adjustments that are done through superior-inferior adjustments of position, and are viewed mainly as a fine tuning process once the quality of the G is established.
Soundpost position is 2-3mm medial to the lateral terminus of the bridge foot. The superior edge of the SP is 1.5mm inferior to the bridge foot.
Vertical alignment (viewed through the end button hole) is judged relative to the neck block. Horizontal alignment is judged relative to the rib mitres when viewed through the bass side ff.