You've got it right. A moderate diet of +P ammo isn't going to harm your wife's 3913. Lots of people have run a steady diet of +P stuff through their alloy framed Smiths with no problem, but I tend to err on the side of caution when it comes to alloy and so I usually practice with standard stuff.
+P+ pressures can be wildly variable and so are suspect. I avoid that ammo altogether.