Ideally, now would be an excellent time to lock the rate because they are at historic 50 year lows. Rates are expected to go up in the near future. You might even want to explore closing earlier to take advantage of these fantastic interest rates. You will need a signed contract signed to lock your loan.
Also check with at least 3 lenders and find the one that you are most comfortable with and has the best program for you. Make sure you know whether they are going to portfolio the loan (keep it in house) or sell it on the secondary market. I also assume you are pre-approved so you know your spending limits prior to purchasing. Good luck on your purchase!
Typically, long-term locks are cost prohibitive unless there is reason to believe that a substantial upswing in rates is imminent. There is alot of caution and volatility in the market right now, new home sales aren't exactly stellar and unemployment is still very high. Based on those factors I wouldn't lose sleep on rates moving too much higher in the next several months. Generally, the summer months aren't kind to the equity markets anyway, which is good thing for treasuries and will help keep rates low.