No, taking a DT into a fight usually isnt a really good idea. It models weight as well as drag, so you have a considerably lowered performance on your plane.
Drop tanks are simply secondary weapons - like rockets or bombs. Hit backspace untill they are selected in the ammo counter, then B (or Fire Secondary) to drop them.
I generally use DT to increase my range. That means, to go on an intercept where i know i might have to fly a while, i take around 75-100% (depending on plane always!) and a DT.
If i up at a base to head to one of our own thats capped for defense, ill usually take 25-50% and a DT, dropping the tank as soon as i get there. I dont expect to have to fly far (or live long) after dropping of tank.
I usually drop my tank when getting into a fight. There are exceptions, as to when i think the guy i met might just run anyway or I have a lot of alt and will only BnZ a few passes before i keep going on.
Generally, id say it is better to drop the tank, experience will tell you after a while, when it is save to keep it.