Our vet said wait 2 days after application before bathing the dogs.. but nothing about waiting 2 days after bathing to apply it. I'm looking at the paper that came in the box, and it doesn't say anything about applying before or after a bath?? It just says it is waterproof, and this: "Fipronil collects in the oils of the skin and hair follicles, and continues to be released from hair follicles onto the skin and coat, resulting in long-lasting activity against fleas, ticks and chewing lice"
Maybe that's why your vet said wait a few days after the bath - to give the oils a chance to build up a bit again?