Four retirement planning rules millennials can break (or bend)

Retirement planning is full of rules. Some are helpful. Some are unreasonable. Nearly all attempt to put a one-size-fits-all shirt on bank accounts and lifestyles of many different sizes.

Breaking some retirement rules might give you a guilty conscience, but it shouldn't screw up your future. Here are four of them you can break, or at the very least, bend to your needs.

1. Save 15 percent every year for retirement: Many sources confirm this is a reasonable target, including Fidelity in its latest guidelines.

But it's probably not reasonable to believe you will be able save a steady amount every year, or that you will be able to save 15 percent of your income right out of the gate. Do you want to know how much I saved for retirement my first year out of college? It's an easy answer, the same one I would give if you asked about the following year: $0.

What I'm suggesting is not to throw this rule away, but to understand you may need to work up to it. Give yourself a pass if you can't hit the 15 percent target every single year — like, say, when you are young (or when you are not young, but your kids are, and their preschool tuition makes college tuition look cheap).

Tip: Use a retirement calculator to figure out how much you should be saving, then save more than that when times are flush. Being overambitious when you have extra cash will help make up for the years when you can't save enough.

2. Pick a target-date fund named after the year you plan to retire: Using a target-date fund to save for retirement isn't a rule, but it might as well be: By some estimates, 90 percent of 401(k) contributions will flow into these funds by 2019.

I'm 33, so a 2050 fund would put me right at a retirement age of 67. The 2050 funds from Vanguard, Fidelity and T. Rowe Price currently are all invested in a mix of roughly 90 percent stocks and 10 percent bonds, so it might seem like they are the same. But as I approach retirement, the funds' glide paths — how that investment mix changes over time — start to differ. In 2030, when I have 20 years until retirement, the Fidelity fund will still contain 90 percent stocks. The Vanguard fund will hold about 83 percent in stocks, and T. Rowe Price just 71 percent. None of those allocations are inherently bad, but there is likely one that better reflects how I would like to be invested at that time.

The problem: Many 401(k)s only offer target-date funds from one fund provider, which means I may be stuck with just one choice for a 2050 fund.

Tip: Select a fund not based on the year in its name, but on how it invests. You can find a fund's glide path on the fund company's website. If it takes too much risk for your comfort, look at a fund with an earlier year. If it takes too little, stretch to a later year. The other, and potentially better, option is to build and manage your own portfolio — target-date funds can be expensive, anyway.

3. Subtract your age from 100 to determine your asset allocation: The resulting number, legend says, is the percentage of your retirement portfolio that should be invested in stocks. The rest goes into bonds. As you can see above, the target-date funds I mentioned are already breaking this rule. Here is why: As life expectancies tick up, investing in stocks for longer can provide the growth your money needs to tick up with them.

This rule also takes literally nothing outside of your age into account, and it assumes a standard retirement age. But maybe you want to work well into your 70s, or you have bought into the promise that using Facebook to sell workout DVDs, essential oils or oysters with pearls inside — I swear this is a thing — means retirement at 40.

4. Build an emergency fund before saving for retirement: An emergency fund is important, but having the full recommended three-to-six months of expenses stashed away is less so, especially if putting that cash together means you will miss out on 401(k) matching dollars. That match is a guaranteed return. (You can use a 401(k) calculator to see just how valuable that is.)

Tip: Just $500 in the bank is enough to cover many sudden expenses. You can go back and add more once you have contributed enough to your 401(k) to grab the full match.