I don't even go that far, HH, simply because this is what happens 99% of the time:
Zep to ML CEN 123.45: "ML CEN Zep 641 departed Styx 62, tracking 415, climbing FL210, estimate ZEPPY 78."
ML CEN to Zep: "Zep 641, No IFR traffic, squawk 8898, approaching flight level 180 contact CEN 121.50 for airways clearance."
That 1% of the time when things are slightly different, ATC have your flight plan, which says you want to fly at FL210. They know that you need a clearance above FL180...thus we set the preselector to 18,000 to make sure we don't punch controlled airspace in case we forget to contact them for a clearance, or we are on the appropriate area frequency and ATC have got other things on their mind and forget to give us our clearance. When the altimeters go "ping", we'll remind ATC we need a clearance if they haven't already given us one.
