if it's running it is taking up ram and, whether or not it's a significant amount, processing power
during startup an app could require more out of the device to get set up and ready for use, but tbh in sc's case it has to refresh often anyways. so unless you expect to snap every 2 minutes for a period of time just kill it
the only apps i ever have open are the ones i'm using at that time (ie music & texting) and sometimes web pages that i want to read later but don't want to bookmark