Invested assassin builds (More than 10 levels) tend to have abysmal saves. While Slippery Mind would not completely cure that, it'll help support the incredibly low will saves of roguish assassins much like Shadowdancer at 7, Swashbuckler at 17, and Vigilante with Cause of Vigilance at 13.
My suggestion is to add Slippery Mind as a class feat at level 13.
Slippery Mind for the Assassin Class
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