I disagree here...a fishpoled line will produce no more 'bowing' than a line tied on directly. The crotch still has to exert the same forces regardless of how the line is tied. However, if a fishpoled line is passed around the trunk a few times before it's tied to the base, it will add some rigidity to the tree.
edit: I rethought this. The only way that the fishpoled line will produce more downward force on the crotch is if the pull is near to vertical so that the 'doubled' line will give somewhat of a mechanical advantage. Given that a pull line should pulled from at least a tree length away, and that the pull point will be below the top of the tree (unless it's a spar) the angle will almost always be more acute than 45 degrees (more so for a back leaner), also given the friction in the crotch, the MA will be very little above 1.