Probably the most important thing to know is that withholding taxes is only one aspect to consider when looking at a tax-efficient portfolio. Yes, by holding your international stocks in your taxable account you will be able to claim back a portion of the withholding taxes on your income tax form as "Foreign Tax Credit" as withholding taxes qualify as Non-business income. But are you consider the benefits of holding your Canadian portion in the taxable account so you can benefit from eligible dividend rates? Generally speaking, foreign dividends are taxed at your marginal income tax rate.
The true math depends on the value of your parents' portfolio, their asset allocation, their income needs, province, and some other factors in their personal situation.
Also, if their RRSP is sizeable, they are not making many new contributions, and their desired US holdings are large it may well be worth it to learn the Gambit. There are a few tutorials online that can help - although reading about it can make it seem overwhelming, it's really not very difficult in practice. Having a US dollar RRSP is the easiest way to benefit from withholding tax rules.