What are you being quoted for a 60 day vs. 45 or 30? Chris makes good points above, and if rates go up in the next couple of weeks than you'll probably be matching the rate you could get on the 45 or 30 for what you could get now on the 60. Personally I don't think they'll get much lower than they are now, but if the loan size is large and an 1/8 or 1/4 saves you a lot each month than that's your call.
Another thing to consider is if it's going to for sure close on April 29. If you lock for 60 days now and have some last minute delays than you might have to pay for an extension. So in some cases it makes sense to wait to lock until you know the end is in sight. How is the process going so far and are you fairly sure you'll be closing by then (well, at least from what the bank has been telling you.)?