I asked the same question on a couple of different forums and I have a 550. It was recommended in heavy traffic, or when going through intersections: keep the rpms up above 4k as you need some umph to keep from being road kill - due to duffus's (duffie??) who change lanes without looking or (god forbid) blow through a red light withouth noticing you!!!
So, I tend to keep it in 3rd gear, under 40 mph, which is about 3800 rpm, while tooling through 30-40 mph typical traffic I get. Hope that helps.
Linda in Colorado