Depending on your priorities, if you are replacing/trading in those tires for another size I'd highly recommend considering the 225/40R18 option on those wheels. It's really close to the same outer diameter of the 215/45R18 and is a much better performance option. If ride comfort outweighs cornering ability then stick with the OEM option. If you ever like some spirited sessions in the twisties - go with the 225/40R18. It's a nice squared setup and will give you a nice shoulder even with a high performance summer tire as well as a little curb protection. Despite what TireRack told you about recommended width - I assure you from experience that size works well. I also ran some winter specific tires with an aggressive tread pattern on these wheels in that same 225/40R18 size.
In regards to the shoulder of the tire questions, that really depends most on the design of the tire. I've seen the "edge" you refer to in both summer and all seasons. The amount of stretch plays into this but most of it is the tread pattern. Many all seasons have a pretty rounded shoulder as well. The Continental ExtremeContact DWS and BFGoodrich KDW2 both have a pretty aggressive shoulder. I went with the Hankook EVO V12 option on my 17s and really like the results at a pretty good price point. If price isn't an issue - you cannot go wrong with the Michelin Pilot Super Sports for a great balance of performance, comfort, and lifespan.