The problem is that moves in money market rates lag the move in interest rates overall because they still have inventory that needs to roll off - in this case at relatively higher rates. Once that inventory rolls off, they will have to replace it with lower yielding paper which will begin lowering the interest rate they pay. If you are trying to decide whether to lock in for 6 months at, say, 5% or simply keep your cash in SWVXX currently yielding 5.14% the decision isn't that simple. Over the course of the next six months it is entirely possible (and maybe likely) that the average return on SWVXX will be LESS than 5% and you will find yourself with less money at the end.
Simply parking your money in what is currently yielding the most can cost you over time. Having a comprehensive fixed income plan that you can stick to is a better idea. Does that mean the plan should never change? Of course not, but the planning exercise will help hone your judgment.