First, for anyone unfamiliar with ruching, it's when fabric is gathered evenly to create a rippled effect. On swimsuits, it tends to add a 1940s starlet vibe as well as overall texture. Ruching can be a great thing if used properly and located in the right place on a garment.
In the case of swimwear, suits generally are ruched across the bust in a horizontal pattern to create bombshell allure. But horizontal lines across the midsection are not anyone's best friend.
Stay away from ruching (or any embellishment, for that matter) across the tummy area. It's going to have the opposite effect by drawing attention to the area. It's like celebrities wearing huge sunglasses and hats to shield themselves from attention -- the first thing we do is notice them.
Try going for something that has ruching on the bust or upper area of the suit to pull focus up and away from problem areas.
If you're set on ruching across the tummy, make sure it doesn't add poufiness.