Yes, I'd accept another school just before May 1 to make sure you have something locked down. But there's always "summer melt" to enrollment so you may very well get off the wait list. The *best* thing you can do if you're interested in Smith is to let them know, "If I'm offered a spot, I will come."
Losing a deposit at another school would sting, no doubt. But in the context of the cost of four years of education it's trivial. (And maybe with a wheedle and pretty please about getting in off a waitlist, the other school might take pity and refund it. Seems to me that getting in off a wait list is a better reason than most, it's not just a "change of mind.")
As others have said, odds of getting merit aid off the wait list are vanishingly remote. However, I know someone who got in off the waitlist and got outstanding need-based aid.