Agreed. I just had some 38mm tires on a bike/wheel that originally had 23mm. That was as wide as I could go without running into a clearance problem with the chainstays. Check how much clearance you have with the 28 mm tires at the fork and chain/seat stays and at the brakes (if calipers). That should give an indication of about how wide a tire you can use without having it rub anywhere. Every extra 2 mm of tire width will reduce the clearance on each side by about 1 mm.