When I learnt the school that taught me had us taxi out on the lowest tank and then change to the full tank in the run-up bay. Now I am a little wiser I will always start on the tank that I will take off on. Why? Two reasons. 1, If you forget to change the tank and take off on one that doesn't have much if any fuel in it. 2, By running on the tank you will initially fly on you give it the most time to make sure the fuel is flowing and everything is ok.
In flight if I change tank I sit and wait about 30 secounds with my hand ready to switch back the the orginal tank and the same when i switch the fuel pump off.
Thats the great thing about flying you are always learning... anything that doesn't kill you makes your stronger!
Adam.