Microworms are good, but BBS is best. It's absolutely the most wonderful food for growing the best, healthiest fry, hands down.
I suppose it's not *essential* since I've heard of a few breeders who don't use it, but I personally swear by it and devote countless hours during the first month of the fry's lives to hatching it just because I think it's so good for them.
The biggest reason (aside from it's nutrients) that I swear by it is because little fry don't really hunt unless it's right in their face. Microwoms sink and the only fry who find them are those who hang out on the bottom, but a LOT of fry hang out at the water surface and the bbs finds them.