Some tire profiles are more sensitive to rim width than others. Dunlop, for example, says that their 180 slick works fine with a 6.0 to 6.25 inch wide rim. In the past, both Michelin and Pirelli have stated that a 5.5 to 6.0 inch wheel is suitable for either a 180 or a 190 tire width.
That said, I purchased a Dymag with a 6 inch rim so I'm running a 190/50. After previously running a 180/55 on a stock 5.5 inch rim, I'm pleased with the combination.