You can drain canned food to get rid of some of the excess sodium (emphasis on some of the sodium). I do this with tuna fish, canned chicken, etc. as well. Like someone said above, cooking any type of veggie will get rid of most of the nutrients.
However apparently cooking tomatoes actually increases the antioxidant properties in them, but at the same time decreases the vitamin C in them (source study done at Cornell University in 2008). I'm sure there are other variances of this nature.
Personally I'm on a pretty tight budget myself and sometimes I too have to eat canned veggies and fruits, I would prefer whole organic fruits and veggies myself but at least your not eating "highly fat saturated crap outta a box" as my buddy would say.
One further note, I am fan of complex carbs, and both sweet potatoes and yams are excellent sources of these types of carbs. Studies suggest that eating canned sweet potatoes or yams (uncooked obviously) has no bearing on nutritional value (i.e. nutrients are not being lost in the canning process), apparently its almost as good as eating'em fresh. Just my 2 cents.
Best,
TWB