Also, if your vertical tube is slightly cracked around the bottom, it will allow fuel to leak in, effectively nullifying your reserve. You will just plain run out of gas.

You have two tubes, a long and a short one, that the T takes in fuel through. If the long one is cracked below the level of the short one, then it will suck in fuel and you won't have a reserve.

+1 on the switching while riding. Practice knowing what it feels like by turning off your fuel and finding out how to ride with one hand while reaching down and switching the petcock. You should be able to do it by feel and never even have to look down. Become one with the bike......ok time for bed!