For the most part I plan for 160 kts at 10K. 15K is pushing the sweet zone but if winds were in favor it might make sense. The normal aspirated Skymaster likes 9 to 12K. Take a look on Flightaware once in a while. Granted you won't always know what vintage and power the aircraft has but you can figure it out most of the time with a little research.
Here is a good example, same day, round trip. Here we are going west into the wind and empty:
http://flightaware.com/live/flight/N.../KRGK/tracklog
Here we are returning with the wind and full gross weight. Long cruise climb but pick up some speed at altitude:
http://flightaware.com/live/flight/N.../KAZO/tracklog