Another 'ancient' rule of thumb is 'Do not climb unless you are more than 1 hour from landing'.
Some typical 737-400 figures at 48T:
Climb from FL 320 to Fl 360 Fuel Burn 100kg
Cruise at 320 2240kg/hr
Cruise at 360 2100kg/hr
IE Save 100kg for the hour. (min of 40 mins cruise with 20 mins descent). Factor in improved descent distance at idle as well.