Mortgage interest rate movements are as hard to predict as the stock market and no one can really know for certain whether they'll go up or down.
If you have a hunch that rates are on an upward trend then you'll want to consider locking the rate as soon as you are able. Before you decide to lock, make sure that your loan can close within the lock in period. It won't do any good to lock your rate if you can't close during the rate lock period. If you're purchasing a home, review your contract for the estimated closing date to help you choose the right rate lock period. If you are refinancing, in most cases, your loan could close within 30 days. However, if you have any secondary financing on the home that won't be paid off, allow some extra time since we'll need to contact that lender to get their permission.
If you think rates might drop while your loan is being processed, take a risk and let your rate “float” instead of locking.