Can I substitute canned tomatoes for fresh ones when making soft tacos? Should tomatoes be drained, seasoned, added to meat mixture or served with lettuce, sour cream, and shredded cheese?
It really depends on how you're using them.
If you want them for a topping along with your lettuce, cheese etc... they must be fresh.
If you're adding them to your ground meat and cooking them, you're probably better off with a drained can of peeled, chopped/diced tomatoes (convenience wise).
Doing both isn't uncommon.
You could also possibly use the canned tomatoes to make the base of some sort of cooked salsa (more similar to a jarred picante sauce than a fresh salsa).