Keygrip,
The significance of fuel carried also depends on the difference in the fuel price or the fuel price ratio between departure and destination airports.
If you are flying from KOBE to KAPF in your C150, it is worth taking that extra fuel. As the fuel is a lot cheaper at KOBE it outweighs the extra fuel burned and it is still cheaper to tanker it.
For this very purpose most of the manufacturers publish "Fuel Tankering" tables. By using these tables you can calculate whether its worth carrying that extra fuel.
In my present plane (A320 family) charts are give in FCOM 2.05.70.