Remember to compare your current bracket against what you expect your EFFECTIVE tax rate to be in retirement. Which could be much lower than your bracket rate.
No no no no no no no. NO. And again, NO! Your advice is just wrong here. This is the perfect situation to explain why your advice is wrong, too.
The OP said she will work some type of fun job in retirement. This could have a huge range, but there are a few possibilities:
1) The income from that job in retirement will not reach the 15% tax bracket.
In this case, she would be better off putting some of her retirement savings in a traditional IRA, until her expected taxable income filled the 10% bracket (putting the rest of her income in the 15% or higher bracket).
2) The income from that job will completely fill the 10% bracket, but her family will stay in the 15% bracket.
In this case, there isn't a significant difference between a traditional and a Roth, although a Roth allows more savings in a tax-advantaged account. But this is one case where your advice could lead people astray. Someone in the 15% bracket now, who will be in the 15% bracket in retirement, is going to have an effective tax rate less than 15%. But a Roth account is likely better (it is guaranteed to be no worse).
3) The combined income from retirement jobs and retirement income will push her family into the 25% bracket.
In this case, your advice is very, and factually wrong. There is a range of many tens of thousands of dollars in annual income where the effective tax rate is 15% or below but the marginal tax rate is more than 15%. In all of these cases if the OP followed your advice she would pay thousands of dollars in additional tax, because your trite rule is inaccurate.