Although rates are low, and the Fed didn't do anything dramatic at it's recent meetings, rates can and do fluctuate. They may not be rising in any significant manner currently, however, a quarter of a percent or more can have an effect on the Buyers payments and the home they wish to purchase.
You may want to shop rates right now, as I do know of lenders who have rates available on 30 year fixed rate mortgages at 3% -3.25% currently.
Generally, since a locked in rate is only available for a very short period of time, you may be better off locating your new home and solidifying a contract on it and then lock in the rate, as a normal closing period may take 30-45 days depending on the position of all of the parties to the purchase contract, and that's pretty much a normal lock period.
If I can assist you with any other comments or other real estate related issues, please feel free to contact me.
Michael Bellaire, Broker, ABR