If it's collected frequently enough, is the food left on a can any worse than the other stuff that ends up in the bin? I used to make a token effort - but with fortnightly collections and being not supposed to wrap things in the can bin, more thorough effort is required.
Hands up! I should obviously have phrased that to be "separately to other (non-can) rubbish)". I have never made any effort to squash cans because the collecting box we have is ample for the containers we collect without doing so. (And will still last four weeks, i.e. allowing a forgotten-to-put-the-damn-thing-out week.)
I have never had any significant problem with the camping type opener (as described earlier). And no problem at all with the 'cut through the rim' styles. Some are much better on the small cans (tomato paste, pilchards) than other types of opener.