Interesting question. In my use (Lyman's 358091 WC), I've found that seating out, has a bit more accuracy. I'm guessing that the exposed lead helps to align the bullet, in the cylinder throat, as it leaves the case. I have a number of .357 guns, and this is true for all of them.
As the chamber on a .357 is longer by roughly a 1/8", and I'm shooting .38 special loads put up in .38 special length brass, that add'l jump to the cylinder throat may have an effect on alignment. 358091 is a bevel base design and has three lube grooves with one crimping groove and maybe 1/10" of lead exposed above the crimp groove, BTW.
As to accuracy, the effect is only visible at 25 yds, from a rest, and is less than 1/4" in group size....which might well be accounted for by alloy, amount of lube used, casting flaws, case neck tension...you get the idea. In use, from an offhand position, the difference, if any, is negligible.
Hope that helps...but probably just further muddies the waters. Rod