Gown before groom

Not too long ago I was at a popular consignment store and I overheard a young woman talking to her mom. She had been browsing the racks and came across a wedding gown that she really liked. I heard her say to her mom that she was going to purchase it and the mom encouraged her. The whole time they were laughing because she didn't even have a boyfriend, let alone a finance, yet.

Would you purchase your wedding gown before you were engaged?

On another note, has anyone had any luck with finding a gown at a consignment store? Any advice on saving money on wedding dresses?