smoboKapoww1) all you do is push the cart on pl so it doesn't matter if you die.
This can be true, but not 100% of the time in payload. Feel free to die if it accomplishes something, but you should never just be throwing your body at them because you feel like it.
*** sorry if this wasn't clear
Yes, don't throw your life away. In the context of the initial opening of the doors, you basically have no effect on the destruction of the sentry + the pop of the enemy uber in which case it's okay to die.
If you see a good chance to make a good trade (demo/medic/heavy) or get a lot of dmg, kills etc. Then you are free to throw your body at them. There is a lot of grey area that you'll pick up as you play and decision making that is questionably right or wrong. That being said, I make a lot of questionable (even wrong?) decisions but I can dm my way out which makes them seem "ok" (What I like to call tf2pug-syndrome).