Lets say your flying along (b737) at 370 and on the CRZ page you see 372/392 and you would like to climb to 390 but, you want to climb when the max = 395
Better not fly into a mass of warmer air then,your max will go down quicker that it came up.
Have a look at your fcom,under opt/max graph and watch out for temp changes on your flight plan.