The question you have is more of a sort of "long-term" perspective. And there are many websites discussing as such. I cannot discuss more or better. My advice for my clients in my own practice is more of the short-term perspective from an accountant and tax perspective. I will share with you for the discussion purpose of this website.
That is, you want to watch out for the trend and day to day development in the market to protect your retirement assets. We need to look at the trend in terms of what is happening today and this week.
Only in the larger investment portfolio, outside of the retirement accounts, the separation of various sources, as you are interested in looking at, makes sense in investment strategies. That is, if you are looking Brexit which will happen say in 2018, then you divide your retirement investments accordingly to various sources. In a way, if we get relaxed and don't look at our investment in between, we tend to miss out all the key development and changes in between. That strategy is not practical and not prudent for our little retirement accounts. However, if we have a lot of money to invest, we can afford to put some into some outlier investment alternative and put aside to let it ride, to grow or decline.
For the retirement accounts, the day we put it in, we already saved like 20% or higher federal tax and 10% California State income tax. That 30% of instant tax savings is an investment return rate no other alternative can bring to us. This is the advice I give to my client. As long as we don't loose the principal, we are doing pretty good in this IRA or retirement account contribution already. Even if the person purchases CD and keep in the CD, FDIC insured, in the IRA account, for three years, the savings (earning return) is 10% per year already, even if the CD pays like no interest rate on the principal deposit.
If the money market account is within your retirement account like 401(k) held by your employer, it is in the U.S. dollar denomination.