Tom, when you find the right home, in the right location at the right price, that is when you should start worrying about interest rates. It is good that you are aware of the fluctuation in the rates, but don't let it keep you from purchasing the home of your dreams. If you are uncomfortable going with an interest rate above a certain point, you can write that into the purchase contract. This is a common practice, in fact, in my office the contract is not considered complete without this information being provided. It is an important piece of information that should be provided to the seller so they know what your limits are.
Once you've found your home and have opened escrow, then you will be in a position to start looking at the rates and get ready to lock in. As Corey said, a lock on a rate does not last forever and is non-transferable. There is usually a limit as to the number of days you'll have to close escrow and still keep your locked-in rate.
When you write an offer, your Realtor should be able to give you an estimate as to what your costs on the real estate side of the transaction should be. At the same time, your lender should be able to give you a "good faith estimate" that will let you know an estimate as to what your costs will be on the loan side.
Good luck with your home search! Give the Ransom-McKenzie Team a call if you're looking for someone to work with in Riverside!