As a matter of fact yes i did. when i first got the plane, i really did not know about VNAV or LNAV so i used to put the vertical speed in manually. usually my cruising alt would be around FL340. the thing is, is that the higher your alt is, the harder it is for the engines to get to that height. lets say i put in VS 1800, most likely, the planes speed would fall very fast. just keep in mind that the higher you go, the better chances of stalling. now when you program the cdu correctly, id recommend using LNAV/VNAV for the climbing and crusing part of your flight. VNAV will enable you to safely climb.